With a minor change to the code I posted, the button can be used to pause & resume the threads:

#!perl -slw
use strict;
use Time::HiRes qw [ sleep ];
use threads;
use Thread::Queue;

use constant { IDLE => 0, WORK => 1 };

sub work{
    my( $id, $delay, $Q ) = @_;
    my $n = 0;
    my $state = WORK;

    local $SIG{QUIT} = sub{ $state ^= 1 };

    while( sleep( $delay ) && ++$n <= 100 ) {
        sleep 1 while $state == IDLE;
        $Q->enqueue( "$id:$n" );
    }
}

my $Q = new Thread::Queue;

my @threads = map threads->new( \&work, $_, 0.1 * $_, $Q ), 1 .. 2;

require Tk::ProgressBar;

my $mw = MainWindow->new;

my $pb1 = $mw->ProgressBar()->pack();
my $bt1 = $mw->Button(
    -text => 'Pause/resume thread 1',
    -command => sub { $threads[0]->kill('QUIT') }
)->pack;

my $pb2 = $mw->ProgressBar()->pack();
my $bt2 = $mw->Button(
    -text => 'Pause/resume thread 2',
    -command => sub {  $threads[1]->kill('QUIT') }
)->pack;

my $repeat;
$repeat = $mw->repeat( 100 => sub {
    while( $Q->pending ) {
        my( $id, $progress ) = split ':', $Q->dequeue;
        return unless $progress;
        ( $id == 1 ? $pb1 : $pb2 )->value( $progress );
        if( $id == 2 && $progress == 100 ) {
            $repeat->cancel;
            $mw->exit;
        }
    }
} );


$mw->MainLoop;

END{ $_->join for @threads; }
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But that is actually easier to achieve using a simple shared variable.