use 5.14.0;
use Mojo::DOM;
undef $/;
my $dom = Mojo::DOM->new(<DATA>);

my @details = $dom->find("div.details")->each( sub {
    if ($_->find(".details_head")->first =~ /Alter:/) {
        say $_->find(".details_data")->first->text;
    }
});

__DATA__
<div class="details">
<span class="details_head">Name:</span>
<span class="details_data">Frank</span>
</div>

<div class="details">
<span class="details_head">Alter:</span>
<span class="details_data">23</span>
</div>

<div class="details">
<span class="details_head">Figur:</span>
<span class="details_data">sportlich</span>
</div>