#!/usr/bin/perl
use warnings;
use strict;

sub a_half_nibble_is_00 {
  my $str = shift;         # XXX bounds
  my @masks = map {chr $_} ( 3, 12, 48, 192 );  # XXX magic
  local $_;
  return !! grep { (~($str.'')&$_) eq $_ } @masks; # L::U::first
}

my @chars = map { chr $_  }  (0 .. 127 );

for my $str ( @chars) {
  my $flag = a_half_nibble_is_00( $str) ? '*' : '' ;
  printf "%2s%4d >%08b< >%s<$/", $flag, ord($str), ord($str), $str;
}
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Be well,
rir