Not rare at all! Both as unnamed and as named parameter used in DBI + DBD::CSV:

use DBI;
my $dbh = DBI->connect (
    "dbi:CSV",                  # Unnamed, driver
    undef,                      # Unnamed, username
    undef,                      # Unnamed, password
    {   f_ext    => ".csv/r",   # Named and defined
        f_schema => undef,      # Named and undefined
        });
[download]

FWIW all NULL values in a database are refered to through undef, so you'll see a lot of those in both named and unnamed arguments to functions and methods.