my take on it...

#!/usr/bin/perl -w
# AoH_map.pl --- Perlmonks: Counting elements in array of cases
# Link:    https://perlmonks.org/?node_id=11106779
# Author:  <LanX>
# Base:    <tybalt89> https://perlmonks.org/?node_id=11106792
# Created: 28 Sep 2019
# Version: 0.03

use warnings;
use strict;
use Test::More;

sub transform {
    my %prev;
    my %origin;
    return map {
        my $target = $_->{targetL};
        my $origin = $_->{origin};
        if ( my $prev = $prev{$target} ) {
            $prev->{count}  += $_->{count};
            $prev->{origin} .= " $origin"
                unless $origin{$target}{$origin}++;                  #
+ ++ FIXED
            ()                          # skip duplicate
        } else {
            $origin{$target}{$origin} = 1;
            $prev{$target} = { %$_ };   # return first (clone)
        }
    } @_;
}

# ---------- tests


is_deeply (
    [ transform(
        {'targetL' => 'foisonnement', 'origin' => 'AMG', 'count' => '1
+'},
        {'targetL' => 'foisonnement', 'origin' => 'IDBR', 'count' => '
+1'},
        {'targetL' => 'gonfler', 'origin' => 'IWWF', 'count' => '1'},
        {'targetL' => 'due', 'origin' => 'IWWF', 'count' => '1' },
        {'targetL' => 'due', 'origin' => 'IWWF', 'count' => '1' },
      )
    ],
    [
        {'targetL' => 'foisonnement','origin' => 'AMG IDBR','count'=>'
+2'},
        {'targetL' => 'gonfler','origin' => 'IWWF','count' => '1'},
        {'targetL' => 'due','origin' => 'IWWF','count' => '2'},
    ]
);

is_deeply (
    [ transform(
        {'targetL' => 'foisonnement', 'origin' => 'AMG', 'count' => '1
+'},
        {'targetL' => 'foisonnement', 'origin' => 'IDBR', 'count' => '
+1'},
        {'targetL' => 'foisonnement', 'origin' => 'AMG', 'count' => '1
+'},
        {'targetL' => 'foisonnement', 'origin' => 'IDBR', 'count' => '
+1'},
      )
    ],
    [
        {'targetL' => 'foisonnement','origin' => 'AMG IDBR','count'=>'
+4'},
    ]
);



    
done_testing;
[download]

C:/Perl_524/bin\perl.exe  d:/tmp/pm/AoH_map.pl 
ok 1
ok 2
1..2
Compilation finished at Sun Sep 29 00:44:35
[download]

update

added missing ++ and testcase, see reply from rsFalse for details.

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