...
my $b = $mw->Button( -text => 'start / stop',
             -command=> [ \&startstop, $pb, \$status_var, $fromv,
               
   $tov, \%timer ],
....
sub setUpPb {
   my ($status_var, $fromv, $tov) = @_;

   if( $$status_var < $tov ){
       $$status_var++;
   }else{
       $$status_var = $fromv;
   }
} # /setUpPb