Code:
on 1:TEXT:!imdb *:# {
  set %url http://www.imdb.com/find?s=tt;q= $+ $$2-

and/or 

on 1:TEXT:!imdb *:# {
  set %url http://www.imdb.com/find?s=tt;q= $+ $replace($$2-,$chr(32),+)


though I dont know why you are setting the variable, unless it is going to be sent to an alias or something

var %var = data (local variable)
is often better than
set %var data (global variable)