../lib/Config.pm
Config - access Perl configuration option
    use Config;
    if ($Config{'cc'} =~ /gcc/) {
        print "built by gcc\n";
    } 
The Config module contains everything that was available to the
Configure program at Perl build time.  Shell variables from
config.sh are stored in the readonly-variable %Config, indexed by
their names.
Here's a more sophisticated example of using %Config:
    use Config;
    defined $Config{sig_name} || die "No sigs?";
    foreach $name (split(' ', $Config{sig_name})) {
        $signo{$name} = $i;
        $signame[$i] = $name;
        $i++;
    }   
    print "signal #17 = $signame[17]\n";
    if ($signo{ALRM}) { 
        print "SIGALRM is $signo{ALRM}\n";
    }   
This module contains a good example of how to make a variable
readonly to those outside of it.