I've not used these modules, but from looking at them, my $feed = XML::FeedPP->new( $source ); creates a feed object and $parser->parse( $feed ); parses a string or IOhandle. Therefore, you are getting an error because you are giving the parser something unexpected.

Converting the feed object to a string solves your issue:

my $atom = $feed->to_string();
$parser->parse( $atom );
[download]

Output following this fix:

Magnitude 4.9 about Galapagos Triple Junction region
Magnitude 5.0 about 75km ESE of Pangai, Tonga
Magnitude 4.9 about 71km SW of Paita, Peru
Magnitude 4.7 about 68km E of Sarangani, Philippines
Magnitude 4.6 about 60km WNW of Lata, Solomon Islands
Magnitude 4.8 about 170km ESE of City of Saint Paul, Alaska
Magnitude 4.6 about South of the Fiji Islands
Magnitude 4.6 about 98km NNW of Yunaska Island, Alaska
[download]

P.S. I get some Use of uninitialized value in concatenation (.) or string at ... warnings in your rules section.

Oh! I didn't realize I was switching types. Thank you!

Inspired by frozenwithjoy, I played with it a little, arriving at

#!/usr/bin/perl -w

use XML::FeedPP;
use XML::LibXML;
use XML::Rules;
use LWP::Simple;
use strict;
use warnings;

# get the Atom feed from the USGS

my $source = 'http://earthquake.usgs.gov/earthquakes/feed/v1.0/summary
+/4.5_day.atom';
my $feed = XML::FeedPP->new( $source );

# Incorporating a rules section so that we can extract the information
+ from the CDATA section
# and any other specific fields we might need.  First attempt will be 
+to pull the 'Updated'
# field data from the Atom feed.  I tried to follow the example in the
+ CPAN summary but I'm
# clearly lost.  

my @rules = (
         _default => sub {$_[0] => $_[1]->{_content}},
         id => sub {$_[0] => $_[1]->{_content}},
#        author => undef, # ignoring the author because I already know
+ it's the USGS
         author => sub {$_[0] => $_[1]->{_content}},
         updated => sub {print "$_[1]->{updated}\n";
         },
);

my $parser = XML::Rules->new(rules => \@rules);
my $atom = $feed->to_string();
$parser->parse( $atom );

# This section extracts the title field, then performs string manipula
+tions to extract the
# long location data and the magnitude.  Funny that USGS does not have
+ a magnitude field
# in this feed.

foreach my $quake( $feed->get_item() ) {
    my $title = $quake->title();
    my $place = substr($title, 8);
    my $magnitude = substr($title, 2,3);
    my $id = $quake->get( "id");    
    my $update = $quake->get( "updated" );
    my $location = $quake->get( "georss:point");
    print "Magnitude ", $magnitude, " about ", $place, "\n";
    print "id is $id, updated at $update, georss:point is $location \n
+";
}
[download]

Running it,

C:\Users\jkeys>perl c:\myperl\usgsfeed_norules.pl
Use of uninitialized value in concatenation (.) or string at c:\myperl
+\usgsfeed_norules.pl line 25.

Use of uninitialized value in concatenation (.) or string at c:\myperl
+\usgsfeed_norules.pl line 25.

Use of uninitialized value in concatenation (.) or string at c:\myperl
+\usgsfeed_norules.pl line 25.

Use of uninitialized value in concatenation (.) or string at c:\myperl
+\usgsfeed_norules.pl line 25.

Use of uninitialized value in concatenation (.) or string at c:\myperl
+\usgsfeed_norules.pl line 25.

Use of uninitialized value in concatenation (.) or string at c:\myperl
+\usgsfeed_norules.pl line 25.

Use of uninitialized value in concatenation (.) or string at c:\myperl
+\usgsfeed_norules.pl line 25.

Use of uninitialized value in concatenation (.) or string at c:\myperl
+\usgsfeed_norules.pl line 25.

Magnitude 4.9 about 67km SW of Painan, Indonesia
id is urn:earthquake-usgs-gov:us:c000hl79, updated at 2013-06-11T03:00
+:47.746Z, georss:point is -1.8063 100.1636
Magnitude 4.7 about 131km NNE of Calama, Chile
id is urn:earthquake-usgs-gov:us:c000hl5v, updated at 2013-06-11T03:13
+:17.731Z, georss:point is -21.3693 -68.4452
Magnitude 4.9 about Galapagos Triple Junction region
id is urn:earthquake-usgs-gov:us:c000hl36, updated at 2013-06-11T06:39
+:02.261Z, georss:point is 1.3674 -101.2936
Magnitude 5.0 about 75km ESE of Pangai, Tonga
id is urn:earthquake-usgs-gov:us:c000hl2w, updated at 2013-06-11T06:31
+:30.385Z, georss:point is -20.0901 -173.6971
Magnitude 4.9 about 71km SW of Paita, Peru
id is urn:earthquake-usgs-gov:us:c000hkbe, updated at 2013-06-10T22:23
+:52.275Z, georss:point is -5.5453 -81.5723
Magnitude 4.7 about 68km E of Sarangani, Philippines
id is urn:earthquake-usgs-gov:us:c000hk85, updated at 2013-06-10T19:54
+:48.457Z, georss:point is 5.2973 126.0703
Magnitude 4.6 about 60km WNW of Lata, Solomon Islands
id is urn:earthquake-usgs-gov:us:c000hk60, updated at 2013-06-10T16:28
+:39.451Z, georss:point is -10.4972 165.3268
[download]

This provides a few examples of accessors. I didn't attempt to dive into the rules, other than to comment out the $author => undef which quieted some of the uninitialized value warnings.

Hope this helps.

With the other problems solved, let's look at the rules.

1. The _default => sub {$_[0] => $_[1]->{_content}}, is better written as _default => 'content',. There are several builtin rules so if that you need to do with a tag matches one of them, it's better to use the builtin instead of a custom rule.

2. If the rule for a tag matches the _default rule, you don't need the tag-specific rule.

3. The rule for the <updated> tag should be updated => sub {print "$_[1]->{_content}\n";},. You want to print the contents of the tag, not the value of its (nonexistent) attribute named "updated".

4. Once you need to work with more of the data you'll probably replace the rule for <updated> with the builtin rule "content" and specify a custom rule for the <entry> tag. In that rule all the contents of the child tags will be available in the $_[1] hashref as $_[1]->{childtagname}.

Jenda,

Thanks for your suggestions on the rules. I've gotten to the point where I'm trying to extract values out of the CDATA field. I've tried a lot of different ideas (HTML tables, simple HTML extraction, stripping tags, RegExp, etc.) but I think that using the ::Rules engine would simply be the most straightforward. I've read your CPAN writeup on ::Rules (are you the author? Very cool) and studied but I'm not quite sure how to best proceed.

I can extract the CDATA content and end up with a resultant set of tags and values. Your comment leads me to believe that I can create a hash of the tags and values then pick the ones I want. That seems to be the exact discussion in the ::Rules section about addresses, streets, Larry Wall, multiple tags and hashrefs. But I don't understand the discussion in that section, can you expand further?

Your XML::Rules section, quoted below, would seem to be the relevant part.

our %states = (
          AL => 'Alabama',
          AK => 'Alaska',
          ...
        );
        ...
        state => sub {return 'state' => $states{$_[1]->{_content}}; }

 or

        address => sub {
                if (exists $_[1]->{id}) {
                        $sthFetchAddress->execute($_[1]->{id});
                        my $addr = $sthFetchAddress->fetchrow_hashref(
+);
                        $sthFetchAddress->finish();
                        return 'address' => $addr;
                } else {
                        return 'address' => $_[1];
                }
        }
[download]

In XML, these two are equivalent: <foo>&lt;bar/&gt;</foo> and <foo><![CDATA[<bar/>]]></foo>. Thus the content of the <summary> tag is the "<p class="quicksummary"><a href="http://earthquake.usgs...". If you want to split that into pieces you have to pass that string to another HTML or XML parser. It's like a box that, apart from other things, contains another box so after you've opened the outer box, you have to extract the inner box and open it as well.

Jenda
Enoch was right!
Enjoy the last years of Rome.