Simply strut each BrowseEntry, then...

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11139495
use warnings;
use Tk;
use Tk::BrowseEntry;

my $top = MainWindow->new;
my $f = $top->Frame;

my $f1 = $f->Frame->form(-fill => 'x');
$f1->Frame(-width => 150)->pack; # strut
my $c1 = $f1->BrowseEntry(-label => "Misc1:", -width => 0,
  )->pack(-fill => 'x', -expand => 1);
$c1->insert("end", "0");
$c1->insert("end", "LargeButWillStillFitInTopWidgetAsItExpands");

my $f2 = $f->Frame->form(-fill => 'x', -left => $f1);
$f2->Frame(-width => 150)->pack; # strut
my $c2 = $f2->BrowseEntry(-label => "Misc2:", -width => 0,
  )->pack(-fill => 'x', -expand => 1);
$c2->insert("end", "0");
$c2->insert("end", "Large");
$c2->insert("end", "LargeEnoughToNotFitInTopWidget");

$f->pack(-fill => 'x', -expand => 1);

MainLoop;
[download]