It passed through my mind that that was what you were after. The following is probably what you want:

use strict;
use warnings;

use Tk;
use Tk::MatchEntry;

my %data = (
    a => [qw/Abberley Abberton Abbotsley Accrington/],
    b => [qw/Babraham Bacton Bexhill Bradfield/]
    );
my $place;
my $mw = MainWindow->new (-title => "i wish this would work",);
$mw->geometry ("200x70+0+0");

my $label = $mw->label ("type somthing in the box");

my $me = $mw->MatchEntry (
    -textvariable   => \$place,
    -choices    => [keys %data],
    -autosort   => 0,
    -fixedwidth => 0,
    -ignorecase => 1,
    -maxheight  => 10,
    -listwidth  => 240,
    -onecmd => \&repopulate,
    )->pack (-side => 'left');

Tk::MainLoop ();


sub repopulate {
    return unless exists $data{lc $place};

    $me->configure (-choices => $data{lc $place});
}
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