And here is another solution:

#!/usr/bin/perl

use strict;
use warnings;

my @AoH = ({qw/ targetL foisonnement origin AMG count 1 /},
       {qw/ targetL foisonnement origin IDBR count 1 /},
       {qw/ origin IWWF targetL gonfler count 1 /},
       {qw/ origin IWWF targetL due count 1 /},
       {qw/ origin IWWF targetL due count 1 /});
my @AoHfinal =
  ({'targetL' => 'foisonnement', 'origin' => 'AMG IDBR', 'count' => '2
+'},
   {'targetL' => 'gonfler',     'origin' => 'IWWF',     'count' => '1'
+},
   {'targetL' => 'due',         'origin' => 'IWWF',     'count' => '2'
+});

my %out = (); my %seen = ();
my @out = ();
foreach (@AoH) {
  if ($out{$_->{targetL}}) {
    push @{$out{$_->{targetL}}->{origin}}, $_->{origin}
      unless $seen{$_->{targetL}}{$_->{origin}}++;
    $out{$_->{targetL}}{count} += $_->{count};
    } else {
    my $row = {targetL => $_->{targetL}, origin => [$_->{origin}],
           count => $_->{count}};
    $out{$_->{targetL}} = $row; push @out, $row;
    $seen{$_->{targetL}}{$_->{origin}} = 1;
  }
}
$_->{origin} = join(' ', @{$_->{origin}}) for @out;

use Test::More;
is_deeply(\@out, \@AoHfinal);
done_testing();

__END__
[download]

I think that the most notable difference in my solution is keeping the running origin values as an array until the very end, instead of repeating string manipulations. This preserves the order of both the targets and the origins while using only the core language — and a core module to verify the result.