You don't want to (can't actually) scroll the main window directly. You need to insert a scrolled Frame or Pane into the toplevel (main window) then pack your widgets inside that.

use strict;
use warnings;
use Tk;
use Tk::Pane;

my %w;

$w{mw} = MainWindow->new;

$w{mw}->geometry('220x350');

$w{bframe} = $w{mw}->Scrolled(
    'Frame',
    -scrollbars => 'oe'
)->pack(
    -expand => 1,
    -fill   => 'both'
);

for (qw/ this that a_longer_name X cheese verb witty_retort superstiti
+on A..Z whatever/){
    my $bttn = $w{bframe}->Button(
        -text     => "$_",
        -font     => 'times 19',
        -justify  => 'left',
        -compound => 'right',
        -bitmap   => ('error','info','question','warning')[rand 4],
        -anchor   => 'e',
        -padx     => 4,
    )->pack;
    push @{$w{buttons}}, $bttn;
}

$w{mw}->update;

{
    my $width = 0;
    for (@{$w{buttons}}) {
        $width = $_->width if $_->width > $width;
    }
    for (@{$w{buttons}}) {
        $_->configure(-width => $width);
    }
}

MainLoop;
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