Why does `$v = () = split` return 1?

Question

perldoc says "a list assignment in scalar context returns the number of elements on the right-hand side of the list assignment" but when I try this code:

perl -e '$_="aaaaa";print $v=(()=split //)'

The output is 1 which makes me confused. (The answer I expect is 5.)

Can anybody explain this?

Answer 1

According to split documentation:

When assigning to a list, if LIMIT is omitted, or zero, Perl supplies a LIMIT one larger than the number of variables in the list <...>

Since you specify empty list, split only returns 1 result and this number of results is exactly what ends in your variable.

Answer 2

split has some kind of crazy ultra-magic in it that allows it to know when it is on the right hand side of an assignment that has a list on the left hand side, and adjusts its behavior according to the number of items in that list.

This is described in perlfunc as being done "to avoid unnecessary work", but you've found an observable difference in behavior caused by that optimization.

To see some evidence of what happened, run your script through Deparse like this:

perl -MO=Deparse -e '$_="aaaaa";print $v=(()=split //)'

Update: I went looking for the code that implements this, and it's not where I expected it to be. Actually the optimization is performed by the assignment operator (op.c:Perl_newASSIGNOP) . split doesn't know that much about its context.