Question

I was quite surprised to find that the keys function happily works with arrays:

keys HASH
keys ARRAY
keys EXPR

Returns a list consisting of all the keys of the named hash, or the
indices of an array. (In scalar context, returns the number of keys or
indices.)

Is there any benefit in using keys @array instead of 0 .. $#array with respect to memory usage, speed, etc., or are the reasons for this functionality more of a historic origin?

Seeing that keys @array holds up to $[ modification, I’m guessing it’s historic :

$ perl -Mstrict -wE 'local $[=4; my @array="a".."z"; say join ",", keys @array;'
Use of assignment to $[ is deprecated at -e line 1.
4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29

Answer 1

The link you provided actually has one important reason you might use/not use keys:

As a side effect, calling keys() resets the internal interator of the HASH or ARRAY (see each). In particular, calling keys() in void context resets the iterator with no other overhead.

That would cause each to reset to the beginning of the array. Using keys and each with arrays might be important if they ever natively support sparse arrays as a real data-type.

All that said, with so many array-aware language constructs like foreach and join in perl, I can’t remember the last time I used 0..$#array.