while (<FILE>)
{
    my $var_a = $_;
    my $var_b = "_INDEX";
    my $var_c = $_;
    $var_a =~ s/(^[A-Z]{4})(.*)/$1/;
    $comb = $var_a . $var_b;
    open (my $fh, "c:\\my\\dest\\dir\\" . $comb . "found.txt")
         or die "failure to create '$comb': $!\n";
    print $fh $var_c . "content detail for file found\n";
}