Namespace URIs are case sensitive. The namespace your file uses is different to the namespace in the module documentation:

file: http://www.itunes.com/dtds/podcast-1.0.dtd
doc:  http://www.itunes.com/DTDs/Podcast-1.0.dtd
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You can register a new prefix:

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

use XML::RSS::Parser;

my $p = XML::RSS::Parser->new;
$p->register_ns_prefix('lc_itunes',
                       'http://www.itunes.com/dtds/podcast-1.0.dtd');
my $feed = $p->parse_file(*DATA);
for my $item ($feed->query('//item')) {
    say $item->query('lc_itunes:summary')->text_content;
}

__DATA__
<rss xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd"
     xmlns:content="content"
     xmlns="http://purl.org/rss/1.0/"
     xmlns:dc="http://purl.org/dc/elements/1.1/">
     <channel>
         <title>Title</title>
         <link>file:///</link>
         <description/>
         <dc:language>cs_CZ</dc:language>
     </channel>
    <item>
      <guid isPermaLink="false">5460304c-e284-4f84-b4a3-2cccba31fb81</
+guid>
...
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