open(my $in, "<", "input.txt") or die "Can't open input.txt: $!";
open(my $out, ">", "output.txt") or die "Can't open output.txt: $!";
open(my $log, ">>", "my.log") or die "Can't open my.log: $!";
while (<$in>) { # assigns each line in turn to $_
    print $out "Just read in this line: $_";
}
close $in or die "$in: $!";
close $out or die "$out: $!";

print $log "Finished with input.txt and output.txt\n";
close $log;
exit 0;