Here's an attempt at a solution.

This uses Tk::IO so it requires the query to be in a separate process.
I had no luck trying to use ->kill, so what this does is use a closure to ignore any lines from a query that is not the latest from showing up in the list.
Slow queries are simulated with a sleep 1.

Seems to work :)

#!/usr/bin/perl

use strict;
use warnings;
use Tk;
use Tk::IO;
use Tk::ROText;

my $sequence = 0;

my $mw = MainWindow->new;
$mw->geometry( '+349+440' );
$mw->Button(-text => 'Exit', -command => sub {$mw->destroy},
  )->pack(-fill => 'x');
my $entry = $mw->Entry(
  -validate => 'key',
  -validatecommand => \&lookup,
  )->pack(-fill => 'x');
my $text = $mw->ROText(
  )->pack;

MainLoop;

sub lookup
  {
  my $newword = shift;
  my $want = ++$sequence;
  $text->delete('1.0', 'end');
  if( $newword =~ /^\w+\z/ ) # avoid shell error
    {
    Tk::IO->new( -linecommand => sub {
        $want == $sequence and $text->insert(end => shift) },
      )->exec("sleep 1 ; look '$newword' | head -20");
    }
  return 1;
  }
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