I found a way to work WAY around it... there should be a better way, but after searching around, I couldn't find a simple solution. So I cheated by creating a new BrowseEntry and by using a Frame to keep it in the same place, the 'destroy' in there helps with memory management (so you're not creating hundreds of overlapping BrowseEntry's. Here's the code:

use strict;
use Tk;
use Tk::BrowseEntry;
my $mw = MainWindow->new;

# Mainwindow: sizex/y, positionx/y
$mw->geometry("200x200+100+120");
my $shortlist = [qw(a b c d)];
my $longlist = [qw(a b c d e f g h i j k l m n o p r s t u w)];
&create_dropdown($mw);
MainLoop;

sub create_dropdown {
    my $mother  = shift;
    # Create dropdown and another element which shows my selection
    my $dropdown_value;
    my $dropdown;
    my $frame = $mother->Frame->pack;
    $dropdown = dropDown($dropdown,$frame,\$dropdown_value,$longlist);
    
    $mother->Button( -text => "Short List", -command => 
        sub{$dropdown = dropDown($dropdown,$frame,\$dropdown_value,$sh
+ortlist)})->pack;
    $mother->Button( -text => "Long List", -command => 
        sub{$dropdown = dropDown($dropdown,$frame,\$dropdown_value,$lo
+nglist)})->pack;

    return;
}

sub dropDown{
    my ($oldDropdown, $frame,$dropdown_value,$options) = @_;
    $oldDropdown->destroy if $oldDropdown;
    my $dropdown = $frame->BrowseEntry(
        -label => "Label",
        -variable => $dropdown_value,
        -autolimitheight => 1,
        -choices => $options
        );
    $dropdown->grid(-row=>1,-column=>1);
    $dropdown;
}
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