In recent Perl (which 5.10.1 is in this context), you can also use the defined-or operator: 
my $revmod = %{ $_[0] // {} };

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Update: It should work with plain ||, too, in the older versions. Just make sure you never call it with 0 as the argument.

لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: Dereferencing %hash that does not exist. by choroba
in thread Dereferencing %hash that does not exist. by perldarren

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