Hi, Sorry for not providing a proper example at first time. Thank you very much for your kind support. I receive compilation errors while using your code, since i am a new user, i really don't know how to debug. but the following code works fine and i get my required output. just share with you i have pasted the code i am using now. 
open(IN, "<temp.in") || die "\n Can't open file\n $!\n";
open(OUT,">temp.out");

   ## Search for FTN
   my %values;

   # read through the input fileto get the values
   while(<IN>) {
      if(/<FTN ID="FN(\d{3})">(.+?)<\/FTN>/) { #no need to quote <, >,
+ ", =
         $values{$1} = $2;
      }
   }
   close(IN);

   open(IN, "<temp.in") || die "\n Can't open file\n $!\n";

    ## replace X-REF
    while(<IN> ) {
       s/<FTN.+?<\/FTN>//; # get rid of existing <FTN> tags - we alrea
+dy have the values from them
       s/(<X-REF REFID="FN(\d{3})">.*)(<\/X-REF>)/$1<FTN>$values{$2}<\
+/FTN>$3/; # alter the X-REF LINE
       print OUT;
    }

close(IN);
close(OUT);

[download]
Once again thanks for your support.

edited by ybiC: Replace opening <pre> tag with <code> as per Monastery convention *and* add closing tag for same

In reply to Re: Re: with better example by texuser74
in thread replacing text in specific tags by texuser74

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