Just a couple of lines of the input demonstrating a match and a fail would help. Try replacing the use of $1 with something other than a special variable, I typically use $line. Then see how you get on.

Cheers,
R.

.................................
.................................
s_abc_in(1450) <= s_abc_out(324); # this is my first match
s_abc_in(1451 <= s_abc_out(1450);
.................................
[download]

After the first match , the if condition should execute and place lines of for loop. if it fails the remaining lines of the whole original file should go into the new file. This is my requirement.

Your main problem is using $1. If you used strictures this would be a fatal error, stopping you treading on your own toes :)

use strict; use warnings;

my $label = 'owt';
while (my $line = <DATA>) {
  if ($line =~ m/s_abc_in\(\d+\).*\s*<=\s*s_abc_out\(\d+\).*/){
    for (1..6){
      my $j = $_ + 1;
      print "a_abc_in\_rrfa_$label($j) <= s_abc_out\_rrfa_$label($_);\
+n";
    }
  } else {
    print  $line; 
  }
}

__DATA__
.................................
s_abc_in(1450) <= s_abc_out(324); # this is my first match
an unmatched line
s_abc_in(1451) <= s_abc_out(1450);
unmatched line, as are the ...... lines
................................
[download]

Cheers,
R.

Pereant, qui ante nos nostra dixerunt!