Nothing jumps out at me either. But an easy change would be to open each file just once at the beginning, rather than every time you access it. Aside from moving the open() statements, the only change I think this entails is doing a

at the beginning of the loop that traverses the records in @REFFILELIST.

You might get a speed improvement by restructuring your script to only read through LARGEFILE once. But this changes the order of your output, and maybe you need the output in the order your script provides.

This is not relevant to your problem as far as I can tell, but if you plan to develop your Perl skills, you might want to develop the habit of using three-argument open() and lexical file handles everywhere (like you did for your output file)