Inserting null values into a float4 field using Perl DBI

Greetings!

Would anyone mind educating me about how to properly 
insert null values into a postgresql float4 field
using perl DBI?  I keep getting 0 in the field, instead 
of a NULL value as I would expect.

Background: 

I'm using placeholders with DBI to read in data from a 
file.  I do this a lot, as in this example:

my $sth = $dbh->prepare_cached( q{
INSERT INTO mytable (field1, field2, field3) VALUES (?, ?, ?)        })  ;
 open (FH, "< $path")           or die "Unable to open $path: $!";     while (<FH>) {            chop;            my
($field1,$field2, $field3) = split /,/;            $sth->execute($field1, $field2, $field3);        }        close
(FH);


The sample file I'm reading has some Null values
and looks like this:

1,2,3
4,,6
2,,6
7,8,
12,13,14

However, these values turn up in postgres table as 0, not NULL.
When I do a select on the data read in with DBI, I get:

1,2,3
4,0,6
2,0,6
7,8,0
12,13,14

Would anyone mind telling me what I'm missing and/or point me to 
an example of how to do this properly.  I'm using DBD::Pg 0.93 and 
postgres 7.02 on both NT and Linux.  Thanks!

Dan