my $b = 7 ;              # Number of bits to worry about
  my $p = ($b + 1) >> 1 ;  # Number of pairs of bits to consider
  my $cnt = 0 ;
  for my $v (0..((2**$b)-1)) {
    my $m = 0b11 ;
    for (1..$p) {
      if ($v & $m) { $m <<= 2 ;      }
              else { $cnt++ ; last ; } ;
    } ;
  } ;

##</code><code>##

  ...
  my $iv  = 0 ; for (1..$p) { $iv = ($iv << 2) + 1 ; } ;
  my $cnt = $iv ;
  for my $v ($iv..((2**$b)-1)) {
    ...

##</code><code>##

  my $f = "%0".($b + ($b & 1))."b" ;
  my $cnt = 0 ;
  $cnt += sprintf($f, $_) =~ m/^(?:01|10|11)*00/ for 0..((2**$b)-1) ;

##</code><code>##

  my $f = "%0".(($b + ($b & 1)) >> 2)."X" ;
  my $cnt = 0 ;
  $cnt += sprintf($f, $_) =~ m/[012348C]/ for 0..((2**$b)-1) ;