my $previous_line = undef; # INITIALLY, THERE IS NO 'PREVIOUS LINE' while ( my $current_line = ) { next if defined($previous_line) && (substr($current_line, 10, 2) eq substr(previous_line, 10, 2) ) # OR WHATEVER-IT-IS ... ; << "ooh, we survived!!" so, do something magical >> $previous_line = $current_line; }