How to bind a null value while using insert statements

Hi,

I have the following insert statement

insert into abc (a,b,c) values (?,?,?);

when i execute this insert (in perl) i get the following error

Cannot bind a reference (ARRAY(0x925069c)) ((null))

This is because value of column a is defined but null
So how should in bind null value in my insert statement?

Thanks,
Harpreet