A couple of solutions.

1. Use placeholders:

my $sth = $dbh->prepare("insert test(id, name) values(?, ?)");
$sth->execute($id, $name);
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If $id is undef then a correct NULL value will be passed to the data server (which from the error message would appear to be Sybase or MS-SQL).

An alternative solution:

$id = 'NULL' unless defined($id);
$dbh->do("insert test(id, name) values($id, \"$name\")");
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The latter solution is much inferior to the placeholder solution, and is incomplete and bug-prone (i.e. if $name is NULL instead of $id you need to handle this differently, not to mention quoting issues with strings...)

Michael