I'm guessing by "non 8 letter words" you mean all words that don't have exactly 8 characters? If so, open an output file and loop through the input file doing this inside the loop: print OUTFILE $_ if length $_ != 8;

update oh wait, you want to remove everything except 8-letter words, (or actually create a new file that doesn't contain them). For that you'd do as above but change != to ==.