my $verify_port = sub {
    my $port = shift;
    if ($port) {
        if ($port > MAX_PORT or $port < MIN_PORT){
        return TRUE;
        }
    }
    else {
        $moduleInput{-port} = 123;
        return FALSE;
    }
    };

##</code><code>##

# I ~love~ readonly. Primarily because you can write stuff like:
# print "$MIN_TCP_PORT < x < $MAX_TCP_PORT\n";
use Readonly;

Readonly my $DEFAULT_NTP_PORT => 123;
Readonly my $MIN_TCP_PORT     => 1;
Readonly my $MAX_TCP_PORT     => 65535;

# in case of success: return a 'true' value   (1 or a useful value)
# in case of error:   return a 'false' value  (0 or undef)
sub verify_port {
  my $value = shift;

  # user provided input should be viewed as toxic - do not
  # trust until proven to contain a sane value
  if (defined $value and $value =~ /\A (\d+) \z/xms) {
    my $port = $1;

    if ($MIN_TCP_PORT <= $port and $port <= $MAX_TCP_PORT) {
      return 1;  # or TRUE or $port
    }
  }

  return 0;  # or FALSE or undef
}

# get a port value from somewhere, e.g. Getopt::Long
# use a default value if no value has been provided
my $input = $opt_ntp_port // $DEFAULT_NTP_PORT;

# - usage a -
if (verify_port($input)) {
  (do stuff with $input)
}
else {
  (issue warning / bail out)
}

# - usage b -
verify_port($input)
  or die;

# - usage c -
# (possible if verify_port() returns $port)
my $ntp_port = verify_port($input)
  or die 'Invalid ntp port!';