to make this clearer for beginners:
- in Perl a sub returns the result of the last statement¹
- to avoid unwanted results a last statement can be an "empty" return (recommended by PBP)
- an empty list () as last statement is faster because two opcodes for return are avoided
- a LABEL: without following code is even faster because a new statement is enforced without any opcode
- "_" is a legal identifier but has no magic meaning as label, IOW it's a convention
I wouldn't call the effects undocumented, just an idiomatic approach (and micro optimization)
FWIW using a label can have unwanted global side-effects, because a nested sub could have a goto _;
some experiments:
$ perl -MO=Concise,ret,empty,label -e'sub ret {return}; sub empty {()}
+; sub label {LABLE:};'
main::ret:
4 <1> leavesub[1 ref] K/REFC,1 ->(end)
- <@> lineseq KP ->4
1 <;> nextstate(main 2 -e:1) v ->2
3 <@> return K ->4
2 <0> pushmark s ->3
main::empty:
7 <1> leavesub[1 ref] K/REFC,1 ->(end)
- <@> lineseq KP ->7
5 <;> nextstate(main 4 -e:1) v ->6
6 <0> stub P ->7
main::label:
9 <1> leavesub[1 ref] K/REFC,1 ->(end)
8 <;> nextstate(LABLE: main 6 -e:1) P ->9
-e syntax OK
UPDATE
¹) jdporter pointed out: you say "last statement", but you mean "last expression evaluated". Not the same thing!
well yes not every statement is an expression leading to a value, and it's the last in the current code flow which counts (think if/else), not in the sub.
That's what the docs say:
If no return is found and if the last statement is an expression, its value is returned. If the last statement is a loop control structure like a foreach or a while, the returned value is unspecified. The empty sub returns the empty list |