Most likely, you want to send it a NULL value. You should be using DBI placeholders instead of interpolating data into a string, just to avoid SQL interpolation problems. See DBI for placeholders and http://bobby-tables.com/ for why.

my $sth_insert = $dbh->prepare(<<'SQL');
    insert into mytable (desc,compid)
                 values (?,   ?)
SQL
my ($description, $compid) = ("Some description", undef);
$sth_insert->execute( $description, $compid );
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Update: Actually set the values to insert

I am already using placeholders in my code, so I know that is not the issue.

If you want to enter a specific integer, just pass it on to the database.

If not, construct your query in a way that simply doesn't enter any value for the serial column. For example if the table has columns id and name, and id is serial, simply write something like

my $sth = $dbh->prepare('INSERT INTO your_table(name) VALUES (?)');
$sth->execute($your_name);
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the integer that gets used doesn't matter to me it just has to be unique for each entry. I have not thought about not passing anything to anything to that column in the database.

It's all in the excellent manual: datatype serial (also available in PDF )

Note that:

Prior to PostgreSQL 7.3, serial implied UNIQUE. This is no longer automatic. If you wish a serial column to have a unique constraint or be a primary key, it must now be specified, just like any other data type.

(Btw: a hint regarding the database name: call it postgresql, postgres, or even Pg, but calling it 'Postgre' makes the same impression as writing PERL instead of Perl).