SQL SUBSTRING

i need to do a substring in SQL extracting a field of varying size.  I am
pulling this from a flat file.  I can get it to work if i specify an actual
length like this. 

C+ SELECT COUNT(*) INTO :CNTHLD FROM MIKELIB/PIFCORP 
C+ WHERE SUBSTR(PIFLD,1,:5) =  :#ID   

HOWEVER, the length of the substring is of varying length

i have tried to do something like this

C+ SELECT COUNT(*) INTO :CNTHLD FROM MIKELIB/PIFCORP 
C+ WHERE SUBSTR(PIFLD,1,:#LEN) =  :#ID               

but it doesn't like the variable for the third parm.  

Does anyone know how i can accomplish this.  

thanks

mike
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