use warnings;
use strict;
use XML::Twig;

my $str = '
<Foo>
    <Validation byCluster="true" byOtherPop="false" byHapMap="true" by
+1000G="true"/>
</Foo>
';

my $t = XML::Twig->new(
    twig_handlers => { Validation => \&valid },
);
$t->parse($str);

sub valid {
    my ($t, $tag) = @_;
    my %atts = %{ $tag->atts() };
    for my $att (keys %atts) {
        print "$att\n" if $atts{$att} eq 'true';
    }
}

__END__

by1000G
byCluster
byHapMap
[download]

UPDATE: runrig /msg'd me that only 'true' should be chosen.

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);
[download]

for //Validation/@*[.='true'] echo name() ;
[download]