#!/usr/local/bin/perl -w
my $inblock = 0;

while (<>) {
 if ($inblock ) {
  if (/^\*END/ ) {
   $inblock = 0;
  }
  else {
   s/expression_to_find/replace_with/g;   # e.g.
  }
 }
 elsif ( /^\*DESCRIPTION ) {
  $inblock = 1;
 }
 print;   # print $_ implicitly
}