Where is your .pm file located?

Try copying it to your site/lib directory, which usually is C:\Perl\site\lib on ActiveState.

It is strange that this happens, since both the parent and child should share the same working directory. Is it that you have the second script on a different directory, and you chdir to it before launching it? If this is the case then don't chdir and do

system('perl dir/other.pl');
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and it should work.