I found something.
PerlGuru + something about randomizing array + foreach+splice+last
'If any part of LIST is an array, "foreach" will get very confused
> if you add or remove elements within the loop body, for example with
> "splice". So don't do that.'
> Your testing seems to prove that this is a special case, certainly a
> very useful one.
> Please explain what makes it special.
The 'last' makes it special. By exiting the loop there is no
opportunity for confusion.
But I did not find see "Anyway, don't do it, even with "last"" advices :(