file1 is common_modify.vhd which has following lines of codes 
    sum_1_SUM0_0 : XOR3
      port map(A => b_c, B => a_c, C => c_c, Y => sum_c);
     
    a_pad : INBUF
      port map(PAD => a, Y => a_c);
    sum_1_CO0_i : MAJ3
      port map(A => a_c, B => c_c, C => b_c, Y => N_5);

##</code><code>##

file2 is nets.txt which has following lines of codes 
a_c
b_c
c_c

##</code><code>##

file3 is enabled_nets.txt which has following lines of codes 
a_e
b_e
c_e

##</code><code>##

open (IN1, "<common_modify.vhd") or die;
open (IN2, "<nets.txt") or die;
open (IN3, "<enabled_nets.txt") or die;
open (OUT, ">output_file.vhd") or die;
my @nets = <IN2>;
my @enabled_nets = <IN3>;
while (<IN1>) {
	foreach $i (0..$#nets){
	print OUT if (s/\=\>\s+$nets[$i]\,/\=\> 

$enabled_nets[$i]\,/);
	}
}

##</code><code>##

sum_1_SUM0_0 : XOR3
      port map(A => b_e, B => a_e, C => c_e, Y => sum_c);
     
    a_pad : INBUF
      port map(PAD => a, Y => a_c);
    sum_1_CO0_i : MAJ3
      port map(A => a_e, B => c_e, C => b_e, Y => N_5);