XML::Twig atts is one way:

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.