Something you may want to pay attention to: you didn't check your open for success. Make it a habit:

open FH, "<$filename" or die "can't open $filename: $!"; Also, skip the open business entirely (and process as many files as you want or if none given optionally take input from STDIN) by simply using <> to read: while (my $line = <>){ Lastly.. note that the author wanted the four y values in separate arrays..

Makeshifts last the longest.