Problem with sequences

Hi,

I'm writing a script, which has to retrieve a next value from a sequnce. I use 
mod_perl, DBI and DBD::Pg version 0.98.

I notice that when I execute "select nextval('sequence_name'::text) AS 
sname;", via DBI, the value of sequence is updated, but the mod_perl returns 
"DBD::Pg::st execute failed: ERROR:  sequence_name.nextval: you don't have 
permissions to set sequence"

The above command is executed sucssesfuly in psql and returns the desire 
result.

Does anybody has an idea what is the problem ?
Vladimir