This may be a good point to introduce the module Data::Dumper.
This module is one of the most useful modules for this kind of debug.
Code: PERL
#!/usr/bin/perl
use Data::Dumper;
print "Content-type: text/html\n\n";
print "<html><head/><body><pre>\n";
print Dumper(\%ENV);
print "</pre></body></html>\n";
Note: I'm not working in the internet environment so some errors can appear on my code, but the idea is there ;)