G'day kulua,

Welcome to the Monastery.

"I am new to perl."

As a first step, please read "perlintro: Perl introduction for beginners". It's not particularly long and will provide you with a basic understanding of the language. It's also peppered with links to more detailed information and advanced topics: use those as the need arises.

Next, bookmark the "Perl Online Documentation". Initially, I'd suggest just familiarising yourself with the basic sections: it's a reference work; there's no expectation that you should read it in its entirety.

"I search in the internet for perl code . ... What should be the modified code."

Well, I don't know what search criteria you used, but you've posted what looks like some variety of Bourne shell code.

Here's some fully-functional Perl code that achieves your stated objectives using the input data you provided:

#!/usr/bin/env perl

use strict;
use warnings;
use autodie;

my $in_file = 'pm_11132903_input.txt';
my $out_file = 'pm_11132903_output.txt';
my $format = ".%s(%s),\n";

{
    open my $in_fh, '<', $in_file;
    open my $out_fh, '>', $out_file;

    while (<$in_fh>) {
        if (/^(rist_top_\w+)/) {
            printf $out_fh $format, ($1) x 2;
        }
        else {
            print $out_fh $_;
        }
    }
}

# Just for testing and demonstration
print "*** INPUT: $in_file ***\n";
system cat => $in_file;
print "*** OUTPUT: $out_file ***\n";
system cat => $out_file;
[download]

Output:

*** INPUT: pm_11132903_input.txt ***
module abc(
rist_top_tck,
rist_top_tdi,
rist_top_tdo,
*** OUTPUT: pm_11132903_output.txt ***
module abc(
.rist_top_tck(rist_top_tck),
.rist_top_tdi(rist_top_tdi),
.rist_top_tdo(rist_top_tdo),
[download]