FYI, while keys %hash returns the number of keys in scalar context, you can also use the hash itself, the value will be false if the hash is empty and true otherwise (actually 0 when empty, and information on the content otherwise). So if (exists $href->{cow} and %{ $href->{cow} }). Unlike the keys version, scalar %{ $href->{cow} } will not create a new hash (autovivify) if the cow key doesn't exist, but die instead (at least if you forgot to check if the key exists, you'll get an error in the right place).

In reply to Re: keys %{$hash->{$href}} adds $href to the hash if it doesnt exist? by Eily
in thread keys %{$hash->{$href}} adds $href to the hash if it doesnt exist? by nikmit

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