You're wrong about split. It can take a string and interpret it as a regex. You don't need to escape commas in a regex, either. If you want to, fine, but it's not necessary.

Using strict is just going to make the code fail worse. Not that that's a bad thing.

The problem is the single quotes in $in{'$field'}. But please, please read Dominus's article (mentioned above) about why you shouldn't be doing this in the first place.