I get the same error without the int($PM) line.

No, you don't - the error message will be different. Please provide the exact error messages and the exact code used to produce them.

You've also edited your original post without marking it updated. Don't do that, see here for why.

why doesn't it treat the keys like a number?

If $PM is "parent mass", how do you expect Perl to use that as a number?

I suspect your problem is in the input data.

Please read and follow How do I post a question effectively? - please show the exact code used, the exact error message, and the exact sample input used to generate that error.