use Data::Dumper;

use strict;

my $var = [
    {
        'city' => 'New York',
        'name' => 'Bill'
    },
    {
        'city' => 'New York',
        'name' => 'Ronald'
    },
    {
        'city' => 'Boston',
        'name' => 'George'
    }
];

my %result;
push @{$result{$_->{city}}}, $_->{name} foreach (@$var);

print Dumper(\%result);
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How would a module make the transformation much easier? What would you expect a call to the transform function provided by such a module to look like?

As an aside, using map in void context is frowned on. It is generally considered clearer to use a for statement modifier:

push @{$var1->{$_->{city}}}, $_->{name} for @$var;
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Thanks for the 'map' advice.
I like to provide same functionality in Template ( TT2). I do not see an easy way. A module plugin would help. I

I imagine there is some module out there that does it, but writing a general purpose routine to convert an array of key-value pairs into a hash of arrays (HoA) is less than 10 lines of code:

use strict;
use warnings;

sub convertKeyValueArrayToHash {
  my ($aData, $sKeyName, $sValueName) = @_;
  my $hData={};
  foreach (@$aData) {
    my $k = $_->{$sKeyName};
    my $v = $_->{$sValueName};
    push @{$hData->{$k}}, $v;
  }
  return $hData;
}
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And a working demo:

aartist:

As I see it, a module would have to give you a way to specify what you're expecting, what you want, and what mapping you want. I don't see how doing that in a general purpose way is going to be any shorter than the perl code that just does the task. Also, if you do anything other than straight mapping (i.e., computations, etc.) then you're making the interface for a data structure transformation module even more complex. I'm quite willing to be proven wrong, but I don't think I am.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

Perhaps pluggable/dynamic data processing/munging/transforming module? will give you some ideas.