#! /usr/bin/perl

while(<DATA>) {
  chomp($_) ;
  if ( $_ == "aaaa" ) {
    print "$_\n" ;
  }
  else {
    print "OK\n" ;
  }
}

__DATA__
aaaa
bbbb
cccc
dddd

##</code><code>##

aaaa
bbbb
cccc
dddd

##</code><code>##

aaaa
OK
OK
OK