#!usr/bin/perl -w
use strict;
use warnings;

my @input_files=<*.txt>;
my $local_count=0;



foreach my $input_file(@input_files)
{
unless (open(INPUT, $input_file))
{
    print "Cannot open file \"$input_file\"\n\n";
    exit;
}

my $sequence='ABCD';

while (my $line=<INPUT>)
{
    if ($local_count==0){
	my $outfile=$input_file;
	$outfile=~ s/\.txt/\.tag\.txt/gi;
	unless (open (OUTPUT, ">$outfile"))
	{
	    print "Cannot open file \"$outfile\"\n\n";
	    exit;
	}
    }
	chomp $line;
 
     
   if($line=~m/$sequence/i){
      	    $local_count++;
	    print OUTPUT"$sequence\n";
	}
	
    }
}