Prepared statements- variable number of placeholders

Is it possible, and if so, how can I set the number of placeholders in a
prepared statement at runtime?

IE, given something like:

my $sth = $dbh->prepare("INSERT INTO $table ($fields) VALUES (?,?,?));

which is fine if I know how many fields I'm inserting into, but what if
I don't know till runtime how many fields?  How can I put the (?,?,?)
into a variable and have it recognised as placeholders?

Thanks,
Tim Bowden