The thing I somewhat worry about is that if I don't put the generated package in %INC, it doesn't work, it says: Can't locate package Somepackage.pm. If I put the package name into %INC, all is well.

However, what do I put as the value in %INC? Normally, these are filenames pointing back to the original source file. In this case, there is no physical file.

Putting some random value in it ('abc') works fine and I should be ok. Does perl ever use the actual value, expecting it to be a file and thus possibly causing havoc in my application?

Perl itself does not use it so I think I'd just put in a 1 to make it true - in case there is code out there testing $INC{$filename} rather than exists($INC{$filename}).