I was working on this from the OP:

"However, when the hash table is empty, we just call return."

That appears to be confirmed in his code example: 

...
%$hash_ref or return; # just return
...
$key;                 # or last line in sub returns a scalar

[download]

So, %some_hash is ending up as one of: 

( key1 => ($val1), key2 => ($val2) )
( key1 => (),      key2 => ()      )
( key1 => ($val1), key2 => ()      )
( key1 => (),      key2 => ($val2) )

[download]

These will be flattened to: 

( 'key1', $val1, 'key2', $val2 )
( 'key1', 'key2'               )
( 'key1', $val1, 'key2'        )
( 'key1', 'key2', $val2        )

[download]

The first is fine; the second is what the OP had a problem with; the last two are problematic but, assuming he's using warnings, should generate an "Odd number of elements in hash assignment" diagnostic (from perldiag).

I don't think the "comma operator returns the right-most value in the list" comes into play in any of these scenarios: all lists are either empty or only have one value.

Am I missing something with your "return (17, 42);" example?

— Ken

In reply to Re^3: Preventing unintended list expansion inside hash literals. by kcott
in thread Preventing unintended list expansion inside hash literals. by gregory-nisbet

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