japhy has answered the meat of the question, but I'm just going to chip in with a note about your colleague's use warnings; over -w on the shebang line.

use warnings; was introduced with Perl-5.6.0, and can be used in place of the -w switch in #!/usr/bin/perl -w

If you are running - or need to ensure compatibilty with - a version of Perl earlier than 5.6.0, then you should use -w, otherwise it is preferable to use warnings.
See perllexwarn and warnings for more info.

Cheers.

BazB.