What a clever inefficiency! I was trying to find a way to generate a list of numbers that was at least as big as each list, but failed...

With ?: I can save some, and shaving with map$foo,@list tricks I can save some more. This brings the safe version down to 48 characters.

sub Mix {
map{my$i=$_;map$i<@$_?$$_[$i]:(),@_}0..map@$_,@_
}
[download]

That matches what was done unsafely before. But unfortunately for sanity's sake observe the following 33 character entry:

sub Mix {
map{splice@$_,0,1}map@_,map@$_,@_
}
[download]

I have written saner code...

UPDATE
Saved 3 chars on the unsafe example, there is no need for a nice numerical list when I will be just converting the elements...

UPDATE 2
Sheesh. And the safe example can be modified to the rather bizarre 45 character beast:

sub Mix {
map{splice@$_,0,1}map@_,map@$_,@_=map[@$_],@_
}
[download]

UPDATE 3
I give up on shrinking this. However I have 4 variations on the key 33 char sub. In terms of efficiency of execution, the following wins:

sub Mix {
map{map{splice@$_,0,1}(@_)x@$_}@_
}
[download]

So that is (barring the unexpected) my final answer.