@data = ( 10,4,20,2,15,6 );

%slot = ( 'one' => 'unoccupied', 'two' => 'unoccupied' );

my $pm = new Parallel::ForkManager( 2 );

DATA_LOOP:
foreach my $n (@data) {
	if ( $slot{ 'one' } eq 'unoccupied' ) { $slotnow = 'one'; $slot{ 'one' } = 'occupied'; } elsif ( $slot{ 'two' } eq 'unoccupied' ) { $slotnow = 'two'; $slot{ 'two' } = 'occupied'; }
	$pm->start and next DATA_LOOP;
	print $slotnow." sleep($n)\n";
	system( "sleep $n" );
	#print "- $slot{$slotnow} - $n\n";
	$slot{ $slotnow } = 'unoccupied';
	#print Dumper( %slot );
	$pm->finish;
}
$pm->wait_all_children;