Have you downloaded/installed the Instant Client SDK too, which provides the required header files? If so, does ORACLE_HOME point to the proper directory?

Makefile.PL's find_headers() routine looks in a number of places:

    (...)
    my @try = (    # search the ORACLE_HOME we're using first
        # --- Traditional full-install locations
        "$OH/rdbms/public", # prefer public over others
        "$OH/rdbms",
        "$OH/plsql", # oratypes.h sometimes here (eg HPUX 11.23 Itaniu
+m Oracle 9.2.0),
        # --- Oracle SDK Instant Client locations
        "$OH/sdk/include",
        # --- Oracle RPM Instant Client locations
        "/usr/include/oracle/$client_version_full/client", # Instant C
+lient for RedHat FC4
        "/usr/include/oracle/$client_version_trim/client", # Instant C
+lient for RedHat FC4
        "/include/oracle/$client_version_full/client", # Instant Clien
+t for RedHat FC3
        "/include/oracle/$client_version_trim/client", # Instant Clien
+t for RedHat FC3
    );
    unshift @try, $::opt_h if $::opt_h;
    @try = grep { -d $_ } @try;
 
    my %h_file;
    find( sub {
    return unless /^o(ci.{3,4}|ratypes)\.h$/i;
    my $dir = $File::Find::dir;
    $h_file{$_} ||= $dir; # record first one found
    print "Found $dir/$_\n" if $::opt_d;
    }, @try);
    (...)
[download]

If none of the predefined search locations exist, the @try list passed to find() will be empty, which causes File::Find to issue the "invalid top directory" error you're seeing...