#!/usr/bin/perl -nw
use constant ID => 'id_tag'; # ID what you searching
use constant ID_LENGTH => length(ID);

if (substr($_, 0, ID_LENGTH) eq ID ) {
  print "found:\n$_";
  exit;
}

##</code><code>##

# [id_tag] [byte offset]
id_tag1 0
id_tag2 123
id_tag3 463
...