6 as 8-bit unsigned =   00000110 =   6 as 8-bit 2's comp
226 as 8-bit unsigned =   11100010 = -30 as 8-bit 2's comp
                        + --------
232 as 8-bit unsigned =   11101000 = -24 as 8-bit 2's comp

##</code><code>##

my $x = 0b11100010;  # 226

$x = -( ( ~$x + 1 ) & 0xFF ) if $x & 0x80;
  -or-
$x -= 0x100 if $x >= 0x80;
  -or-
$x = unpack('c', pack('C', $x));

say $x;  # -30