using arrays as variable, not as field

I read the doc on
http://www.postgresql.org/docs/view.php?version=7.3&idoc=1&file=arrays.h
tml

These doc say you can use array_dims to get the dimension of an array.
But when I try to do some this

select array_dims('{1,2,3,4,5}');

I get this message:

ERROR:  Cannot accept a constant of type ANYARRAY

I see several examples on "field" arrays, but I need to use a variable
with array type inside pl/pgsql (not inside a table), and I need to know
the length of this array. My instalation is 7.3.1.

Anybody have a tip about that?

Thanks in advance!

--
Fernando O. Papa
DBA