Quotes for value of <skip:> directive.

I'm playing around with the <skip:> directive and I've noticed
something interesting that I can't explain to myself. 

Here is my script:

------ Start Script ------
use strict;
use warnings;

$::RD_TRACE = 1;

use Parse::RecDescent;

my $grammar = q{

   input:  number(s) { $return = $item{ number } } | <error>

   number: <skip: '\.*'> /\d+/ 

};

my $parser = new Parse::RecDescent($grammar);

my $test_string = qq{1.2.3.5.8};

print join( "\n", @{ $parser -> input( $test_string ) } );
------ End Script ------

This script works great. However, if I change the value of the skip
directive so that it uses double quotes instead of single quotes:

<skip: "\.*">

the grammar fails to parse the input. However, if I put square
brackets around the escaped dot:

<skip: "[\.]*">

the grammar starts working again:

How does this work this way?

Thanks,

Carl