rucker is exactly right. "test" is a four-letter word because when you use it you're almost always likely to run into some sort of name collision, whether it's in Perl, or in the shell.

Note that when you ask perl to require Test, you end up with the Test.pm module that is included with the Perl distribution:

$ perl -le 'use Test; print $INC{"Test.pm"}'
/usr/lib/perl5/5.6.1/Test.pm
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conv