use warnings;
use strict;

use CGI::Simple;

my $query = '&keyname=value&keyname2=value2&keyname2=value3';
my $c = CGI::Simple->new($query);
for my $p ($c->param) {
  printf "%s: %s\n", $p, join(',', $c->param($p));
}

__END__

keyname: value
keyname2: value2,value3