%$hash_ref or return;

##</code><code>##

%$hash_ref or return undef;

##</code><code>##

keys %$hash_ref; # reset iterator

##</code><code>##

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

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

18:30 >