Probably something like that:
#!/usr/bin/env perl
use 5.010;
use strict;
use warnings;
use Data::Dumper;
use local::lib 'lib';
use XML::Twig;
my $string = <<EOF;
<Person name="Brian">
<Phones>
<Phone name="cell" number="5551112233" />
<Phone name="cell" number="5557778899" />
<Phone name="home" number="5554441122" />
</Phones>
</Person>
EOF
my $twig = XML::Twig->new;
$twig->parse($string);
my @nodes = $twig->findnodes('/Person/Phones/Phone');
my @cellphones_nrs = map {
$_->att('number')
}
grep {
defined $_->att('name')
&& $_->att('name') eq 'cell'
&& defined $_->att('number')
} @nodes;
say "Cellphone: $cellphones_nrs[-1]"
if @cellphones_nrs;
Best regards
McA