If I wanted to do this as a one-off, then I'd use XML-PYX and a shell one-liner like this:

pyx xcard.xml | perl -nle '/^\((\S+)/ && print $1' | sort -u
[download]

To do it programmatically, I'd probably use XML-SAX:

#!/usr/bin/perl -w

use warnings;
use strict;

use XML::SAX::ParserFactory;

my $handler = TagListHandler->new;
my $parser  = XML::SAX::ParserFactory->parser(Handler => $handler);

my $filename = shift or die "No filename\n";

my $tags = $parser->parse_uri($filename);

print "$_\n" foreach @$tags;

exit;


package TagListHandler;

use base qw(XML::SAX::Base);

sub start_element {
    my($self, $data) = @_;
    $self->{tag}->{ $data->{Name} } = 1;
}

sub end_document {
    my($self) = @_;
    my @tags = sort keys %{ $self->{tag} || {} };
    return \@tags;
}
[download]

If you install XML::SAX you'll get a pure Perl parser so you'll also want to install a faster parser module like XML-SAX-Expat.