I installed the module and went looking into the documentation, and somewhere it says
Please refer to 'Google Search AJAX API' documentation for complete list of arguments for Google Search.
There I found that there is the rsz argument
This optional argument supplies the number of results that the application would like to receive. A value of small indicates a small result set size or 4 results. A value of large indicates a large result set or 8 results. If this argument is not supplied, a value of small is assumed.
If you read it, you'll also find key=your-key
my $res = REST::Google::Search->new(
q => 'Larry Wall',
rsz => 'large', # get 8 results per request