How to include a Perl file?

I need to include a Perl library and I'm trying a very basic example to check if I've got the concept right.
This is the content of my file CustomPerl.pl, which is in the graphics folder.

sub mytest {
return "Hello world";
}

And this is what I put in the Body of a question in Lighthouse Studio:

[% Begin Unverified Perl
require '../graphics/CustomPerl.pl';
return mytest();
End Unverified %]

When I test the question I just get "Sawtooth Error # 132". What am I missing here? Do I need to install Perl on my PC?