(my $file = "$module.pm") =~ s[::][/]g;

Note the "=~", not "=". The worst thing about this is it will compile it just fine with it as an equals sign {1}, and simply do the s/// on the $_ variable, and then assign it's return value to $file. Not fun trying to locate that sort of bug.

{1} Yes, even with use strict and warnings on.