use warnings; use strict; my $read_mid1 = 'Reads.fna'; my $read_mid1_correct = 'Reads_correct.fa'; open ( my $input_fh, "<", $read_mid1 ); open ( my $output_fh, ">", $read_mid1_correct); while (my $line = <$input_fh> ) { unless ($line =~ /^>/) { $line =~ s/^/ACGAGTGCGT/; print $output_fh $line; } } close ( $input_fh ); close ( $output_fh );