Welcome back to Perl!

While you were away, the best practice for using open has improved and it is now strongly recommended to use lexical filehandles and the three-argument version of open. In other words, instead of saying

open (FH, "< $file") or die "Can't open $file for read: $!";
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it's better to use

open (my $fh, "<", $file) or die "Can't open $file for read: $!";
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In the new version, $fh is scoped like a regular variable (in the old way, FH is global) and passing < and $file as separate parameters helps to prevent some types of potential attacks by malicious users.

Also, a couple other general tips, which hopefully won't add to your confusion:

• To read in all the lines of your zipcode file, you can use my @zipcodes = <$fh>; instead of the while loop.
• You can combine all the zipcodes into a single regular expression with my $zip_re = join '|', @zipcodes; and then test each $line against that single regex instead of looping through @zipcodes each time.