The reason for the speed difference is because if (%hash) {} ends being implemented internally as

    sv = sv_newmortal();
    if (HvFILL((HV*)hv)) 
        Perl_sv_setpvf(aTHX_ sv, "%ld/%ld",
                (long)HvFILL(hv), (long)HvMAX(hv) + 1);
    else
        sv_setiv(sv, 0);
[download]

So what happens is a new SV is created, its then populated with a string using something like sprintf.

This can be compared against the keys %hash option where an extra opcode is executed, BUT that opcode involves creating an SvIV only and therefore requires no memory allocation, no conversion of longs to strings, etc.

So the bottom line is that if you are concerned about speed use the keys form. In Perl 5.10 we will try to make this an internal optimisation, (internally using if (keys %foo) when the user typed if (%foo) )but its not exactly priority, at least not for me :-).

Update: Well, I gave it a try just to see what was involved, and before I knew it I was sending off patches. So theres a half decent chance this will be fixed in perl 5.10

Pardon my ignorance (I don't know much about the perl internals) but I was under the impression that there was something like boolean context. So shouldn't the hash in if (%hash) {} be able to tell that it was used in boolean context and thus react accordingly? This should then be an easy patch, just defining a different behaviour for %hash in boolean (in contrast to scalar) context.

-- Hofmator

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Well there is no G_BOOL defined, so i dont think internally we have a boolean context. I think what you are thinking of is overloading, which has a bool conversion operator, but the scalarification of a hash in a conditional doesn't involve overloading.

---
$world=~s/war/peace/g

sweet! I really should get familiar with the perl source. I've read through perlgust illustrated a few times and cracked out a little XS code, but maybe it's time to start getting interested in the perl core. Being able to roll an optimisation into the perl codebase - now thats cool!