If you're up to using a different library, then:

use warnings;
use strict;

use XML::Rules;

my $xml = <<XML;
<root>
<Validation byCluster="true" byOtherPop="false" byHapMap="true" by1000
+G="true"/>
</root>
XML

my @rules = (
  Validation  => sub {
    my $d = $_[1];
    my @v = grep  { $d->{$_} eq 'true' } keys %$d;
    print join(",", @v), "\n";
  },
  _default => undef,
);

my $xr = XML::Rules->new( rules => \@rules );
$xr->parse($xml);
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