use strict;
use threads;
use threads::shared;

# actually, maybe Gtk2.
# I'm still not acquainted with Gtk2, so use Tk as example
use Tk;

my $flag : shared;
my $dm = threads->new(\&daemon);

my $mw = Tk::MainWindow->new();

my $but_start = $mw->Button(
  -text    => 'start',
  -command => sub{
    lock $flag;
    cond_signal $flag;
  }
)->pack;

my $but_stop = $mw->Button(
  -text    => 'stop',
  -command => sub{}
)->pack;

#########################################
# the main background thread
#########################################
sub daemon {
  while (1) {
    {
      lock $flag;
      cond_wait $flag;
    }
    
    # do work while active
    my $worker = threads->new(\&work);
    $worker->join();
  }
}

#########################################
# a worker thread that acutally do things
#########################################
sub work {
  system "strange long time work";
}