I'll definitly win the 'ugly one' contest with this one...
Its only advantage : it leaves the arrays passed as arguments untouched, (I feel that someone could make it smaller too)

The most bothering thing for me (apart from the nasty $i handling)is the stop test :
last unless (join'',@n)
Too long, and to weird, anyone to help me improve this ?

sub Mix2 {
my (@r,@n);
my $i=-1;
while($i++>-2){
        @n=map{$_->[$i]}@_;
        @r=(@r,@n);
        last unless (join'',@n)
}
@r;
}
[download]

UPDATE :
To save one char and make it even uglier,
You can whange while($i++>-2) to while($i+++2)
(and my $i=-1; to my $i--; for another one)

"Only Bad Coders Badly Code In Perl" (OBC2IP)