use strict;
use warnings;

my $data = <<IN;
.abc(pqr)
.abc(abc_p)
IN

my $a = qr{ [ .(] }xms;
my $b = qr{  abc | abc_p  }xms;

open my $fh, '<', \$data;

while (my $line = <$fh>) {

    if ($line =~ m{ $a $b }xms) {
        for my $i (1 .. 2) {
            (my $c = $line) =~ s{ ($a $b) }{${1}_$i}xmsg;
            print $c;
        }
    } else {
        print $line;
    }
}