I'm not sure I understand your code, but why not just arrange for $taxis->taxis to immediately return if another instance is already running?

my $RUNNING = 0;
$mw->repeat( 50 => sub { $taxis->taxis(\$RUNNING) unless $RUNNING} ];
MainLoop;

package taxis;
sub taxis
{
   my ($self, $r_running, @whatever, ) = @_;
   $mw->update;   
   $$r_running = 1;
   # do other stuff that takes a while
   $$r_running = 0;
   return;
}
[download]

Of course, this must be modified if $taxis->taxis is recursive.

sub taxis
{
   my ($self, $r_running, @whatever, ) = @_;
   $mw->update;   
   $$r_running = 1;
   # other stuff
   my $null = 0;
   $self->taxis(\$null, $whatever); 
   $$r_running = 0;
   return;
}
[download]

....I think.....

--Bob Niederman, http://bob-n.com

All code given here is UNTESTED unless otherwise stated.