I tried exactly that and I got reference errors. It's very confusing, but the code I posted works fine with 'use strict'. Here is the complete code so far. You are the second person to tell me that it won't work under use strict, but it does here.

#!/usr/bin/perl -w

use strict;
use Tk;

use diagnostics; # Just for testing

my $wintitle = 'TkSysinfo';
my %choices = (
        'fs_sub' => 'File Systems',
        'mem_sub' => 'Memory'
);

my $mw = MainWindow->new;
   $mw->title("$wintitle");
   $mw->minsize(600, 400);
   $mw->configure(-background => '#AAAAAA');

my $menus = $mw->Frame(-relief => 'groove',
        -borderwidth => 3,
        -background => '#EEEEEE'
        )->pack(-side => 'top', -fill => 'x');

my $choice_menu = $menus->Menubutton(-text => 'Choose Info',
        -background => '#EEEEEE',
        -activebackground => '#EEEEEE',
        -foreground => '#000000',
        )->pack(-side => 'left');
foreach (keys(%choices)) {
           $choice_menu->command(-label => $choices{$_},
                -activebackground => '#CCCCCC',
                -command => \&$_);
}

   # EXIT
   $choice_menu->separator;
   $choice_menu->command(-label => 'Exit',
        -activebackground => '#CCCCCC',
        -command => sub{$mw->destroy});

# Define updates to main window
sub fs_sub{};
sub mem_sub{};

MainLoop;
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