use Config;
if ($Config{cc} =~ /gcc/) {
    print "This perl was built by GNU C.
";
}
use Config qw(myconfig config_sh config_vars);
print myconfig();    # like perl -V without a pattern
print config_sh();   # gives absolutely everything

config_vars qw/osname osvers archname/;

The configuration mechanism that builds and installs Perl assembles a wealth of information about your system. The Config module exports by default a tied hash variable named %Config, which provides access to over 900 different configuration values. (These values are also available through Perl's -V:PATTERN command-line switch.) Config also provides three functions that give more shell-like access to these values, as shown above. For instance, that last call might print out:

osname='openbsd';
osvers='2.6';
archname='OpenBSD.sparc-openbsd';

The module's online documentation describes the configuration variables and their possible values. Be aware that if you move your perl executable to a system other than the one on which it was built, these values may not reflect the current reality; for instance, if you're running a Linux or a Solaris binary on a BSD system.