Update: This would be more accurate

    parameter    hard coded default
    ------------------------------------------------------
    hostname*    local domain socket
    port         5432
    dbname**     current userid
    user         current userid
    password     ''
    debug        undef
    args         ''
    tty          ''
    timeout      60

*  - "hostname" may be abbreviated to "host"
** - "dbname"   may be abbreviated to "db".
[download]

OK. That explains why scripts that worked on my XP system no longer work on my Vista system. The former had DBD::Pg and the latter DBD::PgPP

The problem is implementation differences between DBD::Pg and DBD::PgPP. The former defaults to making a tcp connection unless you set the host parameter to path, in which case it will try for a unix-domain socket. On Win, it could try for a named-pipe connection. I guess it would just take for someone to write the patch.

(I wonder why the PgPP author deviated and introduced the path parameter. It makes the host default useless.)

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

Looking at the source, it looks as though there technically is no default hostname. Check out the _connect sub, and you'll see that it checks whether hostname is defined to decide whether to try a TCP socket. But looking a few lines up at new, there isn't any default for hostname, it has to be passed as an argument to new():

sub new {
    my ($class, %args) = @_;

    my $self = bless {
        hostname              => $args{hostname},
        path                  => $args{path}     || DEFAULT_UNIX_SOCKE
+T,
        port                  => $args{port}     || DEFAULT_PORT_NUMBE
+R,
#...snip!
[download]

The former defaults to making a tcp connection unless you set the host parameter to path,

Are you sure? The documentation for DBD::Pg says it defaults to using a local domain socket when the host isn't specified (which is what DBD::PgPP does).