Hello gregory-nisbet,

IIUC, a straightforward way to get the desired behaviour is to change this line:

%$hash_ref or return;
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— which, if %$hash_ref is undefined may return either undef or () (the empty list), depending on whether the subroutine is called in scalar or list context — to this:

%$hash_ref or return undef;
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BTW, I get the same results with this line:

keys %$hash_ref; # reset iterator
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commented-out. Why do you think it’s useful here?

Anyway, hope that helps,

Update 1: It seems I missed the point of your question, which is “What's the idiomatic way to guard against unintentionally expanding a non-singleton list into the bodies of your hashes?” I think your own solution, to “use an intermediate scalar variable” is probably the best way.

Update 2: Alternatively, you could combine your suggestion of using the first element with a test for definedness:

18:30 >perl -MData::Dump -wE "sub f { return (17, 42); } my %h = ( key
+1 => (f())[0] // undef ); dd \%h;"
{ key1 => 17 }

18:30 >perl -MData::Dump -wE "sub f { return ();       } my %h = ( key
+1 => (f())[0] // undef ); dd \%h;"
{ key1 => undef }

18:30 >
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