Don’t be flummoxed by the “large” size of the file.   (To Perl, it isn’t large at all.)

You have two kinds of lines you are looking for:   “code” lines, and “decision” lines.   What you care about in the first case is to capture $last_code_seen.   In the second case, if (defined($last_code_seen)) { ..., you want to make your decision.

Perl munches on “several hundred megabyte” files as a midnight snack.