I can't see why the following line of code would throw an interpreter warning (useless use of hash in void context...)

It doesn't. It doesn't even compile. I bet the line is really

my $foo = defined($some_hash{bar}) and $some_hash{bar};
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which is the same thing as

( my $foo = defined($some_hash{bar}) ) and $some_hash{bar};
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where the result of defined is assigned to $foo and $some_hash{bar} never is. Perhaps you want

my $foo = defined($some_hash{bar}) && $some_hash{bar};
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or

my $foo = ( defined($some_hash{bar}) and $some_hash{bar} );
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Take care, or and and have very low precedence.