in reply to map with empty item

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).

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Re^2: map with empty item
by AnomalousMonk (Archbishop) on Mar 15, 2012 at 07:27 UTC
    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.   [...]   The "odd number of elements in hash assignment" in your code resulted from those three undefs corresponding to the non-matching keys... (Emphasis added.)

    The implicit return of a function (such as a map function block) is the value of the last expression evaluated. The statement
        $_->{key} => $_->{value} if($_->{key} =~/^a\d+/)
    will return false (i.e., '', the empty string) if the regex match fails. This is the source of the '' (empty string) key in the output of the map example of the OP. An undef is never generated by the map function of the example; rather, it is generated by the hash constructor trying to pair the '' up with something.

    BTW: The behavior of the incorrect map example code is highly sensitive to the number and order of items in the data array. Try 

    my $data1=[
{key=>'c4',value=>'valc4'},
{key=>'a1',value=>'vala1'},
{key=>'a2',value=>'vala2'},
{key=>'a3',value=>'vala3'},
{key=>'b1',value=>'valb1'},
{key=>'b2',value=>'valb2'},
{key=>'c2',value=>'valc2'},
];

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    and notice there is no longer any "Odd number of elements..." warning nor undef value, and there are now pairings like  'vala2' => 'a3' (value and key reversed, mismatched) in the output.

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Re^2: map with empty item
by remiah (Hermit) on Mar 15, 2012 at 05:49 UTC

    Thanks, thanks, thanks!

    If I care for unmatched patterns with explicit empty list (), It worked fine!

    Thanks for your explanation.

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