=item substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First character is at
-offset C<0> (or whatever you've set C<$[> to (but B<<don't do that>)).
+offset C<0> (or whatever you've set C<$[> to (but B<don't do that>)).
If OFFSET is negative (or more precisely, less than C<$[>), starts
that far back from the end of the string. If LENGTH is omitted, returns
everything through the end of the string. If LENGTH is negative, leaves that