#!usr/bin/perl

use warnings;
use strict;

my $filename = $ARGV[0] or die "no file name given";

open my $fh, '<', $filename or die "opening '$filename': $!";

my $previous_line = '';
while (defined(my $current_line = <$fh>)) {
    chomp $current_line;
    if ($current_line =~ m{ \A [ACGTacgt]+ \z }xms) {
        print "$previous_line\n";
        print "$current_line\n";
        }
    $previous_line = $current_line;
    }
    
close $fh or die "closing '$filename': $!";