b. This way functions that wish to return failure can just use a simple return without worrying about the context in which they were called.

If you answered a: That would only be true in list context.

If you answered c: That’s what happens when the function ends without return being used at all.

If you answered d: That would only be true in scalar context.

c. The deprecated $* flag does double duty, filling the roles of both /s and /m. By using /s, you suppress any settings of that spooky variable, and force your carets and dollars to match only at the ends of the string and not at the ends of the line as well—just as they would if $* weren’t set at all.

If you answered a: /s only makes a dot able to match a newline, and then only if the string actually has a newline in it.

If you answered b: Although the /c modifier is indeed new as of 5.004 (and is used with /g), this has no particular interaction with /s.

If you answered d: /s does more than that.

a. length is a built-in function prototyped as sub length($), and that scalar prototype silently changes aggregates into radically different forms. The scalar sense of a hash is false (0) if it’s empty, otherwise it’s a string representing the fullness of the hash buckets, like 18/32 or 39/64. The length of that string is likely to be 5. Likewise, length(@a) would be 2 if there were 37 elements in @a.

If you answered b: length %HASH is nothing at all like scalar keys %HASH, which is a good bit more useful.

If you answered c: length %HASH is nothing at all like the size of the list of all the keys and values in %HASH.

If you answered d: You probably think it decided there were 37 keys, and that length(37) is 2. Close, but not quite.

a. A defined (but false) 0 value is the proper indication of the end of file for read and sysread.

If you answered b: You’re thinking of the ioctl and fcntl functions, which return this when the C version returned 0, reserving undef for when the C version returns -1. For example, fcntl(STDIN,F_GETFL,1) returns “0 but true” depending on whether and how standard input has been redirected. (The F_GETFL flag can be loaded from the Fcntl.pm module.)

If you answered c: That’s a string of length 1 consisting of the NULL character, whose ord is 0, which is false. The string, however, is true. read doesn’t return strings, but rather byte-counts.

If you answered d: That would signal an I/O error, not normal end of file. The circumfix operator <> returns undef when it reaches end of file, but a normal read does not.

c. A list is not an array, although in many places one may be used for the other. An array has an AV allocated, whereas a list is just some values on a stack somewhere. You cannot alter the length of a list, for example, any more than you could alter a number by saying something like 23++. While an array contains a list, it is not a list itself.

If you answered a: That makes a reference to a newly allocated anonymous array, and populates it with a copy of the contents of @array.

If you answered b: The backslash operator is distributive across a list, and produces a list in return, this being ($s, @a, \%h, &c) in list context. In scalar context, it’s a strange way to get a reference to the function &c.

If you answered d: @array is not a list, but an array.

d. A regular expression (by definition) must be able to determine the next state in the finite automaton without requiring any extra memory to keep around previous state. A pattern /([ab]+)c1/ requires the state machine to remember old states, and thus disqualifies such patterns from being regular expressions in the classic sense of the term.

If you answered a: The mere presence of minimal and maximal repetitions does not disqualify a language from being regular.

If you answered b: Both NFAs and DFAs can be used to solve regular expressions. Given an NFA, a DFA for it can be constructed, and vice versa. For example, classical grep uses an NFA, while classical egrep a DFA. Whether a pattern matches a particular string doesn’t change, but where the match occurs may. In any case, they’re both regular. However, an NFA can also be modified to handle backtracking, while a DFA cannot.

If you answered c: The (?=foo) and (?!foo) constructs no more violate the language’s regularity than ^ and $, which are also zero-width statements.

a. In Perl, the number of arguments is available to a function via the scalar sense of @_, the return context is available via wantarray, and the types of the arguments via ref (if they’re references) and simple pattern matching like /^d+$/ (otherwise). In low-level languages like C++, where you can’t do this, you must resort to overloading of functions.

If you answered b: Actually, Perl does support overloaded operators via use overload, overridden functions as in use Cwd qw!chdir!, and overridden methods via inheritance and polymorphism. It just doesn’t support functions automatically overloaded on parameter signature or return type. Not that such isn’t longed for.

If you answered c: Perl actually does have function prototypes; however, this isn’t used for the traditional sort of prototype checking, but rather for creating functions that exactly emulate Perl’s built-ins, which can implicitly force context conversion or pass-by-reference without the caller being aware.

If you answered d: Just because it’s hard isn’t likely to rule out something from being implemented—someday.

d. The y/// operator is the sed-savvy synonym for tr. That means y(3) would be like tr(3), which would be looking for a second string, as in tr/a-z/A-Z/, tr(a-z)(A-Z), or tr[a-z][A-Z].

If you answered a: Most people don’t call functions with ampersands anymore. If they did, as in &y(), it wouldn’t be so hard.

If you answered b: y isn’t really a function, per se. If it were, you would never see y!abc!xyz!, since proper functions do not like getting banged on that way.

If you answered c: Functions don’t require prototypes in Perl.

b. Surprisingly enough, you have to put both the reverse and the <FH> into scalar context separately for this to work.

If you answered a: Although scalar <FH> did retrieve just the next line, the reverse is still in the list context imposed on it by print, so it takes its list of one element and reverses the order of the list, producing exactly the next line. An expensive way of writing print scalar <FH>.

If you answered c: Although the first use of scalar inhibits the list context being imposed on reverse by print, it doesn’t carry through to change the list context that reverse is imposing on <FH>. So reverse catenates all its arguments and does a byte-for-byte flip on the resulting string.

If you answered d: That reads all lines in FH, then reverses that list of lines and passes the resulting reversed list off to print. This is actually a very useful thing, and simulates tail -r behavior but without the annoying buffer limitations of that utility. Nonetheless, it’s not what we want.

b. The /g state on a global variable is not protected with local. That’ll teach you to stop using locals. Too bad $_ can’t be the target of a my—yet.

If you answered a: However, if you do a while(<>) and forget to first localize $_, you’ll hurt someone above you. That’s because even though foreach implicitly localizes $_, the while (<>) construct does not.

If you answered c: Doing a local on an imported variable is not harmful. Of course, in the case of $_, it’s virtually unnecessary, since $_ is always forced to mean the version in the main package, that is, $main::_.

If you answered d: This looks close to the bizarre phenomenon known as variable suicide, but that only occurs in ancient Perl versions.

d. Perl can use closures with unreachable private data as objects, and C++ doesn’t support closures. Furthermore, C++ does support pointer arithmetic via int *ip = (int*)&object, allowing you to look all over the object. Perl doesn’t have pointer arithmetic. It also doesn’t allow #define private public to change access rights to foreign objects. On the other hand, once you start poking around in /dev/mem, no one is safe.

If you answered b: Both support multiple inheritance.

If you answered c: Exchange “Perl” and “C++” in that answer, and you would be telling the truth. C++ is too primitive to know when an object is no longer in use, because it has no garbage collection system. Perl does.

d. Odd though it appears, this is how it works. Note that because the second is a symbol table lookup, it is disallowed under use strict “refs”. The words global, local, package, symbol table, and dynamic all refer to the kind of variables that local affects, whereas the other sort, those governed by my, are variously known as private, lexical, or scoped variables.

If you answered a: Try again. You’re close.

If you answered b: One is the scoped variable, the other the package variable. Which is which, though?

If you answered c: There is no difference between a package variable and a global variable. All package variables are globals, and vice versa.

a. Dereferencing a string with *{“STR”} is forbidden under the refs stricture, although *{STR} is allowed. This is similar in spirit to the way ${“STR”} is always the symbol table variable, while ${STR} may be the lexical variable. If it’s not a bareword, you’re playing with the symbol table in a particularly dynamic fashion.

If you answered b: Assuming that the expressions don’t get resolved at compile time, this all has to wait until run time. Something like *Foo::varname, however, would be looked up at compile time.

If you answered c: The %Foo:: hash is always the symbol table associated with package Foo; such a hash can hardly be called regular. Both versions actually refer to the same typeglob, although somewhat differently.

If you answered d: Although you can get the symbol table of the Foo package via the %Foo:: hash, you cannot usefully generate new typeglobs (symbols) this way. You could copy old ones into that slot, though, effectively doing the Exporter’s job by hand.

d. If it weren’t for HTML comments, improperly formatted HTML, and tags with interesting data like <SCRIPT>, you could do this. Alas, you cannot. It takes a lot more smarts, and quite frankly, a real parser.

If you answered a: As written, the dot will not cross newline boundaries, and the star is being too greedy. If you add a /s, then yes, it will remove all tags—and a great deal else besides.

If you answered b: It is easy to construct a tag that will cause this to fail, such as <IMG SRC=‘foo.gif’ ALT=“> ”>.

If you answered c: For a good deal of HTML, this will actually work, but it will fail on cases with annoying comments, poorly formatted HTML, and tags like <SCRIPT> and <STYLE>, which can contain things like while (<FH>) {} without those being counted as tags. Comments that will annoy you include <!-- <foo bar = “-->”> which will remove characters when it shouldn’t; it’s just a comment followed by “>. And even something like <!-- <foo bar = ”--> most browsers will get right, but the substitution will not. And if you have improper HTML, you get into even more trouble, like this: <foo bar = “bleh” @> text text text <foo bar = “bleh”>. Here, the .*? will gobble up much more than you thought it would.

a. The indirect object syntax only has a single token lookahead. That means if new is a method, it only grabs the very next token, not the entire following expression. This is why new $obj[23] arg doesn’t work, as well as why print $fh[23] “stuff ” doesn’t work. Mixing notations between the OO and IO notations is perilous. If you always use arrow syntax for method calls, and nothing else, you’ll never be surprised.

If you answered b: If the current package did in fact have its own new function, then this would be the right answer, but for the wrong reasons. Within a class, it might appear to make no difference since the new subroutine would get its argument in $_[0] whether it’s called as a function or a method. However, a method call can use inheritance, while a function call never does. That means esoteric overridden new methods would be duped out of calling their derived class’ constructor first, and we wouldn’t want that to happen, would we?

If you answered c: Perl may be crazy, but it’s not quite that crazy. Yet.

If you answered d: Just because it looks like a unary function doesn’t mean a method call parses like one. You just want it to work this way. If you want that, write that.

c. This is scalar context, not list context, so we have the bistable flip-flop range operator famous in parsing of mail messages, as in $in_body = /^$/ .. eof(). Except for the first time f() returns true, g() is entirely ignored, and f() will be ignored later when g() is evaluated. Double dot is the inclusive range operator; f() and g() will both be evaluated on the same record. If you don’t want that to happen, the exclusive range operator, triple dots, can be used instead. For extra credit, describe this: $bingo = ( a() .. b() ) … ( c() .. d() );

If you answered a: You’d be amazed at how many things in Perl don’t cause syntax errors.

If you answered b: That sounds more like a negated logical xor. A logical xor is !$a != !$b, so you’ve just described !$a == !$b. Interesting, and perhaps even useful, but unrelated to .., our scalar range operator.

If you answered d: That might work in list context, but never in scalar. The list operator .. is a completely different creature than the scalar one. They’re just spelled the same way, kind of like when you can the rusty old can down by the guys’ can just because you can. Context, as always, is critical.

b. Perl keeps track of your variables, whether dynamic or otherwise, and doesn’t free things before you’re done using them.

If you answered a: Even though the reference returned is for all intents and purposes a copy of the original (Perl uses return by reference), the underlying referent has not changed.

If you answered c: Perl seldom stops you from doing what you want to do, and tries very hard to do what you mean to do. This is one of those cases.

If you answered d: Perl is not C or C++.

c. Only lexical variables are truly private, and they will persist even when their block exits if something still cares about them. Thus { my $i = 0; sub next_i { $i++ } sub last_i { --$i } } creates two functions that share a private variable. The $i variable will not be deallocated when its block goes away because the next_i and last_i subroutines need to be able to access it.

If you answered a: Perl is not the Korn shell, nor anything like it. If you tried this, your program probably wouldn’t even compile.

If you answered b: The local operator merely saves the old value of a global variable, restoring that value when the block in which the local occurred exits. Once the subroutine exits, the temporary value is lost. Before then, other functions can access the temporary value of that global variable.

If you answered d: It would be difficult to keep private state in a function otherwise.

d. When the interpreter exits, it first does an exhaustive search looking for anything that it allocated. This allows Perl to be used in embedded and multithreaded applications safely, and furthermore guarantees correctness of object code.

If you answered a: Under the current implementation, the reference-counted garbage collection system won’t notice that the object in $ap’s array cannot be reached, because the array reference itself never has its reference count go to zero.

If you answered b: That would be very bad, because then you could have objects whose class-specific cleanup code didn’t get called ever.

If you answered c: A class’s DESTROY function, or that of its base classes, is called for any cleanup. It is not expected to deallocate memory, however.

a. It has been said that all programs advance to the point of being able to automatically read mail. While not quite there yet (well, without loading a module), Perl will at least automatically send it.

If you answered b: That would be bad. Very Bad. What do you think we are? A shell or something?

If you answered c: It would be improper to run anything at all in the face of such naughtiness.

If you answered d: An appealing idea, though, isn’t it? After all, Perl does possess super(user)powers at this point. You just never know what it might do. In the interests of courtesy, though, Perl stays out of your power supply just as it stays out of your living room.