strange message from pg_dumpall

Hello all,

I got surprised by this message:

-------------------------------------
The program "pg_dump" is needed by pg_dumpall but was not found in the
same directory as "/usr/bin/pg_dumpall".
Check your installation.
--------------------------------------

It's quite strange because I'm quite the pg_* binaries are well
installed:

----------------------------------------
$ ls -la /usr/bin/ | grep pg_
-rwxr-xr-x 1 admin root 16632 Oct 16 17:39 pg_config
-rwxr-xr-x 1 admin root 16636 Oct 16 17:39 pg_controldata
-rwxr-xr-x 1 admin root 24880 Oct 16 17:39 pg_ctl
-rwxr-xr-x 1 admin root 187996 Oct 16 17:39 pg_dump
-rwxr-xr-x 1 admin root 43444 Oct 16 17:39 pg_dumpall
-rwxr-xr-x 1 admin root 23020 Oct 16 17:39 pg_resetxlog
-rwxr-xr-x 1 admin root 84460 Oct 16 17:39 pg_restore
$
----------------------------------------

This is what I do:

system("su postgres -c \"pg_dumpall --clean > some_file\"");

this is run from within a Perl module called from within a Perl script.

I've tried the same command directlly on the shell and it works fine.
But from the perl script it complaints about pg_dump !!!???

I've even tested both commands directlly on the cmd line and they work
properlly !

any ideas ?

thx
joao