Did you forget an explicit return? Forget? No. Omit? Quite likely. Live with it. Cos nothing in this world is going to make me give up a 400% speed gain:

[0] Perl> sub a(){ 1 }; sub b(){ return 1; };; [0] Perl> cmpthese -1,{
+ a=>q[ a() for 1 .. 1000;], b=>q[b() for 1 .. 1 +000;] };; Rate b a b
+ 4668/s -- -80% a 23195/s 397% --
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Just in case some (Perl) newbie doesn't get that subroutines return the value of their last expression and need a keyword to tell them so. No way José.

Your benchmark is rather bogus. The performance "hit" from the return statement is negligible. It's the prototype for no arguments with no return statement that gives a seeming big return (I'm guessing the interpreter is optimizing it to nearly a no-op).

$ perl -E 'use Benchmark qw(timethese cmpthese); sub a() { 1 }; sub b(
+) { return 1 }; cmpthese -1,{ a=>q[ a() for 1 .. 1000;], b=>q[b() for
+ 1 .. 1000;] };'
     Rate    b    a
b  2694/s   -- -88%
a 23209/s 761%   --
$ perl -E 'use Benchmark qw(timethese cmpthese); sub a { 1 }; sub b { 
+return 1 }; cmpthese -1,{ a=>q[ a() for 1 .. 1000;], b=>q[b() for 1 .
+. 1000;] };'
    Rate   b   a
b 2595/s  -- -8%
a 2823/s  9%  --
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Is there a performance gain from not using return in this spot? Yes.

Is it a premature micro-optimization which is unlikely to have anything but a negligible impact the vast majority of real world code? Yes.

Would I consider this performance gain an invalid reason to skip a return statement on any code that hasn't been profiled and clearly shown to benefit from this micro-optimization? Yes.