...
my $seq = <$fh>;   # entire file is now in $seq;
$seq =~ tr/\n//d;  # remove all newlines
open( my $out, ">", "write.txt" ) or die "Cannot create write.txt: $!\n";

my @chunks = split /(..$find..)/, $seq;

my $offset = 0;
for my $chunk ( @chunks ) {
    if ( $chunk =~ /(..)$find(..)/ ) {
        printf( "%s occurs at character offset %d, between %s and %s\n",
                $find, $offset + 2, $1, $2 );
    }
    $offset += length( $chunk );
}