I think this is what you are trying to do. I found it here https://studio.tellme.com/vxml2/ovw/cookies.html#server_cookie (5.2. Retrieving cookies on the server).

# /usr/local/bin/perl -w
use CGI::Cookie;
# retrieve the cookie collection
my %cookies = fetch CGI::Cookie;
# retrieve the user_id cookie from the collection
my $user_id = $cookies{"user_id"};

# lookup some user data (e.g. account balance) based on the user's id
my $balance = GetBalance($user_id);

# return the data to the user
print "text/xml\n\n";
print <<EOF;
<vxml version="2.1">
<form>
   <block>
      <prompt>Your balance is $balance</prompt>
      <goto next="main.vxml"/>
   </block>
</form>
</vxml>
EOF

# retrieve the user's account balance given their id
sub GetBalance
{
  my $user_id = @_;
  return "10000.00";
}
[download]

Good luck!