This won't be speedy but you could use fewer rands by adapting your original approach.

print new_session_id(), $/;

sub new_session_id {
    my $id = '';
    for ( 1 .. 32 ) {
        $id .=  sprintf "%x", rand(16);
    }
    return $id;
}
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You should also see if you've got Digest/Digest::MD5 available on your system. You can do something like:

use Digest::MD5 "md5_hex";

my @user_info = map { $ENV{$_} }
   grep { /USER|REMOTE/ } keys %ENV;

print md5_hex( rand() . join('', @user_info) );
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Don't forget to do a search for unique ID too, there are many points of departure: Google on the pen for "unique id." Also, try vanilla "unique."