The idea (borrowed) here is that transforming the array before processing it makes the alghorithm itself simpler.

my %hash;

for ( map { [ split (/ - /, $_->[0]), [@{$_}[1..$#_]] ] } @nar )
{
    push @{$hash{$_->[0]}->{$_->[1]}}, $_->[2];
}

print Dumper \%hash ;
[download]

Output:

$VAR1 = {
          'fact 2' => {
                        'ParamGrp1' => [
                                         [
                                           'fiu',
                                           'xxx'
                                         ]
                                       ]
                      },
          'fact 1' => {
                        'ParamGrp2' => [
                                         [
                                           'qux',
                                           'bom'
                                         ]
                                       ],
                        'ParamGrp1' => [
                                         [
                                           'foo',
                                           'bar'
                                         ],
                                         [
                                           'fuu',
                                           'gong'
                                         ]
                                       ]
                      }
        };
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Update:
fixed typo pointed out by jdalbec