Something like this? (use a Frame):

use strict;
use warnings;
use Tk;

my $mw = new MainWindow;
$mw->geometry ("400x400");

my $tf = $mw->Frame->pack (
        -side   => "top",
        -expand => 0,
        -fill   => "x",
        );

my $label = $tf->Label (
    -text   => "File?",
    -anchor => "w",
    )->pack (
        -side   => "left",
        -anchor => "w",
        -padx   => 20,
        -pady   => 10,
        );
my $entry = $tf->Entry->pack (
        -side   => "left",
        -anchor => "w",
        -padx   => 20,
        -pady   => 10,
        );
my $button = $tf->Button (
    -text  => "search!",
    -width => 30,
    )->pack (
        -side   => "right",
        -anchor => "e",
        -padx   => 20,
        -pady   => 10,
        );

my $text = $mw->Text (
    -width  => 15,
    -height => 3,
    )->pack (
        -side   => "top",
        -anchor => "nw",
        -padx   => 20,
        -pady   => 10,
        -expand => 1,
        -fill   => "both",
        );

MainLoop;
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