my $x = '255.0.0.0';
print "$x is a valid mask!\n" if validateMask($x);

sub validateMask {
  return                                # return undef if it's not complete
    unless $_[0] =~/^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/;
  $_[0] = unpack (
          "B32",
          pack( 
               "C4", 
               split ( /\./, $_[0] )
              )
         );

  return                                # return undef if we find an invalid
    if $_[0] =~ /01/;                   # mask ( a zero followed by a one )
  return 1;                             # return 1 if everything is okay
}


##</code><code>##

11111111000000000000000000000000 is a valid mask!

##</code><code>##

print "255.0.0.0 is a valid mask!\n" if validateMask('255.0.0.0');

##</code><code>##

Modification of a read-only value attempted at ./idnopheq.pl line 5.

##</code><code>##

my $mask = shift;

##</code><code>##

sub validateMask {
  $_ = shift;
  return unless /^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/;
  unpack('B32', pack('C4', split /\./)) =~ /01/ ? undef : 1;
}