Use Data::Dumper or similar to examine your data whenever you get something weird like this. Your two hashes are very different. First time through (on solution):

$VAR1 = {
          'ParisFR' => '      264.965452179665817',
          'Montana' => '     7596.858969274226183',
          'Correze' => '      639.457965193499604'
        };
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Second time through (problem):

$VAR1 = {
          'ParisFR' => bless( {
                                'unit' => 'kilometre',
                                'values' => {
                                              'kilometre' => '264.9654
+52179666'
                                            }
                              }, 'Class::Measure::Length' ),
          'Montana' => bless( {
                                'unit' => 'kilometre',
                                'values' => {
                                              'kilometre' => '7596.858
+96927423'
                                            }
                              }, 'Class::Measure::Length' ),
          'Correze' => bless( {
                                'unit' => 'kilometre',
                                'values' => {
                                              'kilometre' => '639.4579
+651935'
                                            }
                              }, 'Class::Measure::Length' )
        };
[download]