Does this do what you need?

$string = '0102030405060A0BC';
$fill_char = '0';

if (length($string) % 2 != 0) {
        $string .= $fill_char;
}

@bytes = split(//,$string);

for ($i=0; $i <= $#bytes; $i+=2) {
        print $bytes[$i+1];
        print $bytes[$i];
}
print "\n";
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/\/\averick