use Mac::Plist::Writable;

my $plist = Mac::Plist::Writable->from_plist_file('/path/to/some.plist');
$plist->{'CFBundleIdentifier'} = 'com.bassetsoftware.PerlMonkNode';
Mac::Plist::Writable->add_node($plist, 'NewKey', '42', 'integer');

print Mac::Plist::Writable->to_plist($plist);