question about perl functions

Hi all,
 
I have to do a particular randomized survey, multipage, in which each page
must contain only 1 question (with images, and with a particular layout);
 
I must randomize a block of 20 questions, and then continue the survey. All
these questions must be answered.
 
So, I think that instructions as RANDOMROUTE, RANDOM(survey tag),
SUBSET(survey tag) are not useful because:
 
RANDOMROUTE: I suppose that I cant control which page i come from, and count
the index of my page to learn if I finished my 20 question block or not;
RANDOM: This (I think) works only if I split for example the 20 questions
with 5 questions per page: but this is not my goal;
 
So, following an example by Joel (Interesting use of snippets, 2006/3/2),
I've tried to use perl snippet.
I've created an array with 20 elements, with the idea of using <ROUTE
CONTINUE={&... perl snippet 
in which I want to assign an array index element to tell where to go :
 
<ROUTE CONTINUE="{& print ("page".$myarray[0].".survey")&}"/>
 
Obiouvsly, I have to randomize such array.......
 
And this is my problem: using shuffle or rand() I experienced this message:

Document error

Security exception: 'rand' trapped by operation mask at (eval 83) line 7,
line 16. 

How can I solve problem?
 
 
thanks all,
Luca Notari
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