Not existing, or zero:

    $var = !exists $hash{key} || $hash{key} eq "0" ? 0 : 1;
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Undefined (including not existing) or zero:

    $var = ($hash{key} // 0) eq "0" ? 0 : 1;
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False (which includes zero, undefined, non-existing and empty):

    $var = $hash{key} ? 1 : 0;
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Note the eq "0" instead of == 0; this avoids warnings if the keys contains strings that don't look like a number.