use strict;
use warnings;

use XML::LibXML qw( );

sub format_uri {
    my ($uri) = @_;
    return '' if !defined($uri);
    $uri =~ s/\}/%7D/g;
    return "{$uri}";
}

( my ($xml_file) = @ARGV ) == 1
    or die("usage\n");

my $parser = XML::LibXML->new();
my $doc = $parser->parse_file($xml_file);
my $root = $doc->documentElement();

my %seen;
print "$_\n"
    for
        sort
        grep !$seen{$_}++,
        map { format_uri( $_->namespaceURI() ) . $_->localname() }
        $root->findnodes('//*');
[download]

Update: Now handles the null namespace.