If you lines are sorted by it's ID in the file, then use binary search, overvise try: 

#!/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;
}

[download]

Update: usage: script.pl BIG_FILE.txt

please tell how fast my code on your data, thanks

Update: If your big file contains constant data, then you may create an index file. The format of that file is : 

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

[download]
Thus you are able to search a tag in the index file and fetch searched line using seek

In reply to Re: Specified Line Searching in file! by ccn
in thread Specified Line Searching in file! by Anonymous Monk

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