to suppress unicode's stupid errors.
They are not stupid errors. The errors (in fact warnings - they only become errors from 5.28 onwards) are telling you that you are trying to feed illegal data into the regex engine. It's illegal because the octets you told decode() to interpret as utf8 aren't in fact valid utf8.
How you want to handle this corrupt data is of course your choice depending on what is best for your circumstances. You might want to make decode croak if fed a bad file with FB_CROAK, then go back and delete / fixup any bad files. Or depending on the nature of the corruption of the files, you might like to use FB_DEFAULT to just replace the corrupt bits with REPLACEMENT CHARACTER.
You would only want to use FB_QUIET if you don't mind decode() stopping at the first bad part of the file and ignoring the rest of its contents.