#!/usr/bin/perl -w
use strict;

   while (<DATA>) {
       if (/aaaa$/) {
           local $/ = '%'; # use local here for extra-cleanness ($/ is restored to its previous state when you exit the block)
            while (<DATA>) { print "read: $_\n"; }
       }
   }
__DATA__
XXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXX
XXXXXXXXXXXXaaaa
abcdefghijklm%nopqrstuvw%xyz