I believe the anonymous monk has led you down the wrong path. If Perl wasn't able to find Diff.pm, it would have given you a different error ("Can't locate Diff.pm in @INC").

The problem is that Diff does not export diff. The problem is in Diff, or the problem is in your usage of Diff. We'd have to see Diff to help you more.

Quick Tip #1

Diff.pm should contains something like

@EXPORT_OK = qw( diff );
@ISA = qw( Exporter );
require Exporter;
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Quick Tip #2

Make sure to use use Diff ... if Diff.pm contains package Diff;, or
make sure to use use diff ... if diff.pm contains package diff;.
In Windows, using the wrong case in the use statement will load the module, but will prevent importing from working.