From the transformations documentation:

{s.strip,len}
Return string after removing starting 'len' characters. Parameter 'len' can be positive integer or pseudo-variable holding a positive integer.

I tried to use pvar holding len but got an error.

Jul 11 14:51:37 localhost /usr/sbin/kamailio[16905]: ERROR: pv [pv_trans.c:434]: select cannot get p1

Jul 11 14:51:37 localhost /usr/sbin/kamailio[16905]: ERROR: <core> [lvalue.c:351]: non existing right pvar

This is part of my config file:

$avp(varname) = '1'; <---- pseudo variable holding a positive integer

$avp(result) = $(fU{s.strip,$avp(varname)}); <---- s.strip

How can i use variables with transformation ?