use strict;
use warnings;
use XML::Simple;
use Data::Dumper;

my $data = do {local $/;<DATA>};
my $xml  = XMLin($data);

print Dumper($xml), "\n\n";

for my $id (3,4) {
	print $xml->{gene}{$id}{'label'}, "\n";
}
__DATA__
<?xml version="1.0" ?>
<genes>
	<gene id = "3" label = "gene_of_interest" />
	<gene id = "4" label = "Another_gene_of_interest" />
</genes>