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
................................
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Cheers,
R.