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.