Just an alternate way:

sub interleave {
  my $y;
  while (@{ $_[0] } or @{ $_[1] }) {
      push @$y, shift @{ $_[0] } if @{ $_[0] };
      push @$y, shift @{ $_[1] } if @{ $_[1] };
  }
  return $y;
}
[download]

Ack, what have I done, this will surely lead to endless perl golf!

Ciao,
Gryn

Update: Eek, this doesn't work. Please ignore -- I'm at work and will be a while to fix.

Update: Ok that seems better. Thank you for your patience :)