fixup typo in perlsub.pod

Author: leonerd

pod/perlsub.pod

@@ -489,7 +489,7 @@ at once.
     foo(%params);
 
 As with positional parameters, named parameters may be declared with a
-defaulting expression to provide a value to assign into the varible if the
+defaulting expression to provide a value to assign into the variable if the
 caller does not provide a value.  Any parameters that do not have a
 defaulting expression become mandatory; an exception is raised if the caller
 does not provide an associated value.