Pairing the clumsy Thread::Semaphore with the should-never-have-been-invented "threads signalling" is an extremely convoluted way of controlling a producer thread.

This does what I think you were trying to achieve -- start the thread 'suspended' and allow it to be resumed and suspended -- with rather less fuss and complication:

#! perl -slw
use strict;
use threads;
use threads::shared;
use Thread::Queue;
use Tk;

my $sem :shared = 0;

sub thread {
    my( $Q, $file ) = @_;
    open my $fh, '<', $file or die $!;
    while( <$fh> ) {
        lock $sem; cond_wait( $sem );
        $Q->enqueue( $_ );
    }
}

my $Q = new Thread::Queue;
my $t = async( \&thread, $Q, $ARGV[0] )->detach;

my $mw = MainWindow->new;
   $mw->geometry( "1024x768" );

my $b1; $b1 = $mw->Label->pack( -anchor => 'nw' )->Button(
    -width => 10,
    -text => 'start',  -command => sub{
        if( $sem ) {
            $b1->configure( -text => 'resume' );
            lock $sem; cond_signal( $sem ); $sem = 0;
        }
        else{
            $b1->configure( -text => 'suspend' );
            lock $sem; cond_signal( $sem ); $sem = 1;
        }
    }
)->pack( -side => 'left' );

my $lb = $mw->Scrolled(
    'Listbox', -height => 55, -scrollbars => 'e'
)->pack( -anchor => 's', -fill => 'both' )->Subwidget('scrolled');

my $repeat = $mw->repeat( 1 => sub {
    while( $Q->pending ) {
        $lb->insert( 'end', $Q->dequeue );
        $lb->see( 'end' );
        $mw->update;
        lock $sem; $sem and cond_signal( $sem );
    }
});

$mw->MainLoop;
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Update: Simplified the code a little.