sub munge {
   my $num = shift;
   if ( $number > 1000000 ) {
      return ($number / 1000000).' M';
   }
   if ( $number > 1000 ) {
      return ($number / 1000).' K';
   }
}

my $text = <<END;
Hello 200000 how 12345678 are
you?  I have 35000 dogs.  I 
consume 32000000 hamburgers a day.
END

$text =~ s/(\d+)/munge($1)/g;