My first reaction on seeing require of a .pl file is that it is very old code. I would call it a code smell.

I prefer to put as much code as possible in CPAN-style modules (.pm files), with associated unit tests (.t files) for each module.

The ideal is for commands (.pl files) to be very very short, with almost all automated testing heavy lifting delegated to the module level, leveraging the excellent Perl CPAN module testing tools, such as the prove command. An extreme example of this approach is Perl::Tidy whose perltidy command is essentially just:

use Perl::Tidy;
my $arg_string = undef;
exit Perl::Tidy::perltidy( argv => $arg_string );
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