use warnings;
use strict;

# remember that a byte consists of 2 nibbles and each nibble can be represented as an ASCII character within the stream.
my $stream = '0123??6789??????';
my $lastValidByteIndex;
for my $byteIndex (0 .. length($stream)/2-1) {
    my $byte = substr($stream, $byteIndex*2, 2);
    $lastValidByteIndex = $byteIndex unless $byte eq '??';
}
if (defined($lastValidByteIndex)) {
    print "last valid byte is at index $lastValidByteIndex\n";
} else {
    print "There is no valid data in the stream\n";
}