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'},
];

[download]

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.

In reply to Re^2: map with empty item by AnomalousMonk
in thread map with empty item by remiah

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