my $client_socket = new IO::Socket::INET ( [stuff] )
  or die && return ( [other stuff] );

##</code><code>##

my $checkPort = sub {
    my $port = shift;
    if ($port) {
        if ($port > MAX_PORT or $port < MIN_PORT){
        $error = "Please insert a correct port number (".MIN_PORT." - 
+".MAX_PORT.")";
        return TRUE;
        }
    }
    else {
        $moduleInput{-port} = 123;
        return FALSE;
    }
    };

##</code><code>##

my $port = verify_port($value);

if (defined $port) {
  (do stuff)
}
else {
  (handle error case)
}

sub verify_port {
  my $candidate = shift;

  if ( [suitable] ) {
    return $candidate;
  }
  else {
    return;
  }
}

##</code><code>##

my $port = verify_port($value);

sub verify_port {
  my $candidate = shift;

  if ( [suitable] ) {
    return $candidate;
  }
  else {
    (handle error case)
    (abort execution)
  }
}