Thank you very much for all you guys who helped me.

I came out with a solution some thing like this.

$infile = $ARGV[0];
open(IN, "<$infile");
open(OUT, ">temp.out");
{
   local $/ = '<exp>';
   print OUT scalar <IN>; 
   for (<IN>) {
      s@([\d\D]*?</exp>)@
      my $var = $1;
      $var =~ s#([a-z|A-z]+)#<it>$1</it>#g;
      $var =~ s#<<it>(.*?)</it>>#<$1>#g;
      $var =~ s#</<it>(.*?)</it>>#</$1>#g;
      $var =~ s#&(.*?)<it>(.*?)</it>(.*?)\;#&$1$2$3;#g;
      $var @e; 
      print OUT
   }
}
close(IN);
close(OUT);
[download]

Thanks, raj