Frustrated with do()

Hi,

I'm trying to keep a count of how many times a record was changed.

In the MySQL command line this works perfectly:

UPDATE MyStuff SET cnt=cnt+1,first=7 where last=8;

Field cnt's value is increased by 1 for the correct record.

When I try

$dbh->do("UPDATE MyStuff SET cnt=cnt+1,first=? WHERE last=?", undef, $ff,
$ll);

Field first is updated to $ff for the correct record. However field cnt is
set to 0 (zero).

I can get this behaviour on the MySQL command line with

UPDATE MyStuff SET cnt='cnt+1',first=7 where last=8; (note the quote marks
added)

What is the correct format for the do() statement for what I want to achieve?

Thanks

Nico