The IF EXISTS can be stored in a STATE hash, like so:

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

my $mw = new MainWindow();
my %STATE=(
  'lights'=>0,
  'bell'=>0
);
my %ENABLE=(
  'lights'=>144,
  'bell'=>136,
  'lights&bell' => 144|136, # not sure here
  'nothing' => 128 # enable nothing
);
my $lights;
my $bell;

# Exit-Button
my $message = $mw->Entry()->pack;
$mw->Button(-text=>'End', -command=>sub {exit} )->pack;
$lights = $mw->Button(-text=>'Turn LIGHTS on', -command=>sub {
  # toggle lights
  $STATE{'lights'} = 1 - $STATE{'lights'}; # can also do !=
    # send data to turn on/off lights
    calculateInstruction();
    $lights->configure(-text => 'Turn LIGHTS '.($STATE{'lights'}?'off'
+:'on'));
    updateMessage();
} )->pack;

$bell = $mw->Button(-text=>'Turn BELL on', -command=>sub {
  # toggle lights
  $STATE{'bell'} = 1 - $STATE{'bell'}; # can also do !=
    # send data to turn on/off lights
    calculateInstruction();
    $bell->configure(-text => 'Turn BELL '.($STATE{'bell'}?'off':'on')
+);
    updateMessage();
} )->pack;

sub calculateInstruction{
  my $n;
  if($STATE{'lights'} && $STATE{'bell'}){
    $n = $ENABLE{'lights&bell'}
  }elsif($STATE{'lights'}){
    $n = $ENABLE{'lights'}
  }elsif($STATE{'bell'}){
    $n = $ENABLE{'bell'}
  }else{
    $n = $ENABLE{'nothing'}
  }
  my $instruction = "<f 1108 $n>";
  print "Would send $instruction to serial\n";
#    $port->write($instruction);
#    $port->lookclear();    
}

sub updateMessage{
  my $msg = 'LIGHTS='.$STATE{'lights'}." BELL=".$STATE{'bell'};
  $message->configure(-text => $msg);
}


MainLoop;
[download]

Not sure about the exact codes you need to send, check your manual. Would sending <f 1108 144> then <f 1108 136> turn on both lights and horn? If you send the same command twice, would it toggle the lights? or would this work:

sub calculateInstruction{
  my $instruction;
  if($STATE{'lights'} && $STATE{'bell'}){
    $instruction = "<f 1108 144><f 1108 136>"
  }elsif($STATE{'lights'}){
    $instruction = "<f 1108 128><f 1108 144>"
  }elsif($STATE{'bell'}){
    $instruction = "<f 1108 128><f 1108 136>"
  }else{
    $instruction = "<f 1108 128>"
  }
  print "Would send $instruction to serial\n";
#    $port->write($instruction);
#    $port->lookclear();    
}
[download]

If it is bitwise, like cragapito thinks, then this should work:

sub calculateInstruction{
  my $n = $ENABLE{'nothing'};
  for my $k (keys %STATE){
    $n |= $ENABLE{$k} if ($STATE{$k});
  }
  my $instruction = "<f 1108 $n>";
  print "Would send $instruction to serial\n";
#    $port->write($instruction);
#    $port->lookclear();    
}
[download]