You need to show us where and how you call the &ComboBox subroutine. Follow Abigail-II's advice and read the appropriate man pages.

Rather than sit around and try and second guess what you're doing, I figured I could show you what is going on here. For a quick example of OO in Perl and what is going on here, save the following file as 'Counter.pm':

package Counter;
use strict;
use Carp;

sub new {
    my ($class,$value) = @_;
    $value ||= 0;
    my $self = bless {}, $class;
    croak "Argument to constructor must be positive integer."
        if ! $self->_good_value( $value );
    $self->{_value} = $value;
    return $self;
}

sub value {
    my $self = shift;
    $self->{_value} = $_[0] if $self->_good_value($_[0]);
    return $self->{_value};
}

sub increment_value {
    my $self = shift;
    $self->{_value}++;
}

sub decrement_value {
    my $self = shift;
    $self->{_value}--;
}

sub _good_value {
    my ( $self, $value ) = @_;
    return 0 if ! defined $value;
    $value =~ /^\d+$/;
}

1;
[download]

And then, in the same directory:

#!/usr/bin/perl -w
use strict;

BEGIN {
    unshift @INC, '.'; # add current directory to include path
    use Counter;
}

my $counter = Counter->new;

print $counter->value, "\n";
$counter->increment_value;
print $counter->value, "\n";
$counter->decrement_value;
print $counter->value, "\n";
$counter->value(3);
print $counter->value, "\n";
[download]

The above code is rather simple and not how I would really do it, but it gives you an idea of what is going on. If you don't understand it, you really need to learn Perl's OO system, as what I have presented is very basic.

Cheers,
Ovid

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Just to clarify, the OP doesn't control how subroutine is called. When the end user changes the value of the combo box, Windows sends a message to the main window, which in turn dispatches control to the subroutine.

• You are using $self = shift;, and you are not putting a my in front of it. That means you are using a global variable. If you do the same in SelectedItem or GetString you will get problems, as they will overwrite what is in $self.
• You are using $self as an index into an array. But $self is an object. If you use an object as a number, most of the time, you will get a pretty big number. Perl will happely extend the array @comboboxes for you, but you're likely to run out of memory before Perl has extended the array far enough.

Abigail