None of the code you've posted so far uses Dumper, so seeing all this output from Dumper doesn't help much, since we can't tell what's being Dumped.

If you haven't tried using the perl debugger on this yet (putting perl -d at the start of the command line when you run your script), that's probably what you should do next. Set a break point at the line that causes the error, so that the debugger stops and prompts for input from you when it reaches (but has not yet tried to execute) that line.

When you reach the breakpoint, look at the value of $intersection, and confirm whether or not your hash has a matching key. (According to the error message, it doesn't). Then figure out why $intersection has the value that you see, and/or why the hash doesn't have that as a key.