my %derived = listOfDerivedClasses( 'Element' );

while( <SQL> ) {
    my( $type ) = m[...];
    die "Derived type $type not available" unless exists $derived{ $type };
    push @instances, $type->new();
}

##</code><code>##

while( <SQL> ) {
    my( $type ) = m[...];
    my $inst;
    eval{ $inst = $type->new(); 1 };
    die "Derived type $type not available:'$@'" if $@;
    push @instances, $inst;
}