[Dbix-class] now() - interval ? placeholder trouble

[Bob MacCallum]

Hello,

I can't seem to (>1h banging head against wall) get this search
condition working

$interval = '1 hour';
$rs->search(
{
 birth_time => { '<' => \[ "now() - interval ?", [interval => $interval] ] }
}
)->count

Can anyone tell me what I'm doing wrong, please?

For more, including the runtime error, please see
http://paste.scsys.co.uk/269786

thanks!
Bob.