You have to explicitly specify an empty list (), if you want map to not "return" any items in case the if condition isn't true. Otherwise, map will return undef. 

%ret = map{ $_->{key} =~/^a\d+/ ? ($_->{key} => $_->{value}) : () } @$
+data1;

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The "odd number of elements in hash assignment" in your code resulted from those three undefs corresponding to the non-matching keys b1, b2 and c2. The first two created the hash entry '' => undef, the remaining one created the warning (because hash entries consist of key-value pairs, not a single value).

In reply to Re: map with empty item by Eliya
in thread map with empty item by remiah

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