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