Ok, I rewrote it using strict and warnings and replaced the switch with if/elsif statements. I have $opt declared as a global, but it's throwing the exact same error it was before For some reason I had a brain fart and forgot to use the <code> tags. Sorry about that. It's fixed now.

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

# Variables for emulators
use vars qw($snesCommand $genesisCommand $nesCommand $snesDirectory $n
+esDirectory $genesisDirectory $opt $system $directory $command);

$snesCommand="zsnes";
$genesisCommand="dgen";
$nesCommand="nestra";

$snesDirectory="~/roms/nes/";
$genesisDirectory="~/roms/genesis/Roms/";
$snesDirectory="~/roms/snes/Roms/";

my $mw=MainWindow->new(-title=>"Launcher");

my $systemPick=$mw->Optionmenu(-command=>\&changeList,
                -variable=>\$system);
$systemPick->addOptions("SNES","NES","Genesis");
$opt=$mw->Scrolled("Listbox",-selectmode=>'single',
                -scrollbars=>"se");
my $go=$mw->Button(-text=>"Go for it",
                -command=>\&gogo);

# Geometry
$systemPick->pack();
$opt->pack(-expand=>1,-fill=>"both");
$go->pack();

sub changeList {
    $opt->delete(0,'end');
    if ($system=="SNES") {
        $command=$snesCommand;
        $directory=$snesDirectory;
        my @dir=glob($snesDirectory);
        foreach my $file(@dir) {
            $file=~s/$directory//;
            $opt->insert('end',$file);
        }
    } elsif ($system=="NES") {
        $command=$nesCommand;
        $directory=$nesDirectory;
        my @dir=glob($nesDirectory);
        foreach my $file(@dir) {
            $file=~s/$directory//;
            $opt->insert('end',$file);
        }
    } elsif ($system=="Genesis") {
        $command=$genesisCommand;
        $directory=$genesisDirectory;
        my @dir=glob($genesisDirectory);
        foreach my $file(@dir) {
            $file=~s/$directory//;
            $opt->insert('end',$file);
        }
    }
}


sub gogo {
    my $selected=$opt->get($opt->curselection());
    system("$command $directory$selected");
}

&changeList;
MainLoop;
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