use warnings;
use strict;
use Mojo::DOM;

my $xml = <<'ENDXML';
<foo>3</foo>
<foo>14</foo>
<foo>159</foo>
ENDXML

my $dom = Mojo::DOM->new->xml(1)->parse($xml);
my $i = 1;
$dom->find('foo')->each(sub{ $_->content($i++) });
print $dom->to_string;

__END__

<foo>1</foo>
<foo>2</foo>
<foo>3</foo>