my $sth = $db->prepare_cached( <<SQL ) or die $db->errstr;
insert into t (a, c) values (?, ?)
SQL
$sth->execute( $foo, $bar );

# ...
# sometime later
# ...

my $sth = $db->prepare_cached( <<SQL ) or die $db->errstr;
insert into t (c) values (?)
SQL
$sth->execute( $quux );