It's called XPath. It's used and supported in a wider range of tools/languages/libraries than just XSL. This particular expression means "a div element whose class attribute has the value "event".

> want to parse

Then you can't use handlers, as you need access to more than just a subtree. The following shows how to do it. Using XML::LibXML would simplify the code in such a case, in my opinion.

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use XML::Twig;

my $twig = 'XML::Twig'->new;
$twig->parsefile(shift);

my $root = $twig->root;
for my $header($root->descendants('h3')) {
    my $date = $header->text;
    my @events = $header->next_siblings(sub {
        my ($elt) = @_;
        'div' eq $elt->name && $elt->prev_sibling('h3') == $header
    });
    say join "\t", $date, map $_->text, $_->children for @events;
}
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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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