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
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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