Just store the cost along with the salt and the hash.
Have the user change his password the next time he logs in after you increase the cost.
sub set_passwd {
my ($user, $passwd) = @_;
my $cost = COST;
my $salt = _get_random_salt();
_set_passwd($user, "$cost:$salt:$passwd);
}
sub check_passwd {
my ($user, $submitted_passwd) = @_;
my ($cost, $salt, $passwd) = split /:/, _get_passwd($user);
return hash($submitted_passwd, $salt, $cost) ne $passwd;
}
sub is_passwd_expired {
my ($user) = @_;
my ($cost, $salt, $passwd) = split /:/, _get_passwd($user);
return $cost != COST;
}