my $mask=0;
while (($mask != "27") || ($mask != "28"))
{
        print "Enter right netmask. Only 27 or 28\nNetmask: ";
        $mask = <STDIN>;
        chomp $mask;
        if (($mask != "27") || ($mask != "28")){print "Wrong netmask.\n"}
}

 {
  my $range;
  do {
    $range = <STDIN>;
  } while (not ($range =~ /\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}/));
  ...
}

my $range = <STDIN>;
while ($range !~ /^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/)
{
   print "Wrong IP-range. Enter right IP-range e.g. 192.168.2.0!\nIP-range: ";
   $range = <STDIN>;
}

while ($mask !~ /^\d\d$/)
{
}

$eingabe =~ /^\s*(2[78])\s*$/;
$eingabe = $1;
# wegen \s*, dass heißt, dass auch Platzgelassen werden darf

# Read("eingabe: ",[regex,regex,rege...],"wrong!")
#
# @args = Read(...);
# $args[-1] = nummer der regex die stimmt(von 0)
# $args[0] = Eingabe des Benutzers
# $args[1] = $1, $args[2] = $2, ...

sub Read {
   my $text = shift;
   my $regex = shift;
   my $wrong = shift;
   my($a,$i,@a);

   while (1) {
     print $text;
     chomp($a = <STDIN>);

     $i = 0;
     foreach my$aa (@{$regex}) {
       if ($a =~ /$aa/) {
         push(@a,$a);

         my @aa = $a =~ m/$aa/;
         push(@a,@aa,$i);

         return @a;
       }
       $i++;
     }

     print "$wrong\n" if $wrong;
   }
}