$str = '5B CD 02 01 10';   #Initialize my variable:in my function, this is passed as input
$OldByte_str = substr($str,0,2); #extract the two characters to form a Byte
$PDM_OldBits =  ( "Binary: %bn", $PDM_OldByte );#Convert to binary. Supposed to give '0101 1011'
substr($PDM_OldBit,2,2)= '10';  #Replace two bits.Supposed to give '01**10** 1011'
$NewByte_str= sprintf("0x%x", stringdecimal(arraystring($PDM_OldBits)));#Convert back to Bytes. Supposed to give'6B'
substr($str,0,2)= $PDM_NewByte;#Substitute back into the original Bytes string.Supposed to give: '**6B** CD 02 01 10'

sub stringdecimal {
    return unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
  }

sub arraystring {
 my $string = join('', @_);
 return $string;
}