Hello jasper8989, and welcome to the Monastery!

Your code actually works fine for me. But note that each $zipcode and $line terminates in a newline character. It is better practice to chomp each line of input to remove the terminal newline. It’s possible that your problem stems from a newline mismatch between the lines in the first and second loops.

So, you should begin by verifying that @zipcodes contains the data you expect it to. Print out the contents immediately following the first while loop. See Data::Dumper or (my preference) Data::Dump. Likewise, check the contents of $line on each iteration of the second while loop.

Here are two useful references: Basic debugging checklist and Unbelievably Obvious Debugging Tip.

Update: Added the second reference.

Hope that helps,