I surrender. One way:

Win8 Strawberry 5.8.9.5 (32)  Sun 03/21/2021  4:04:08
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

my $inData = <<INDATA;
int abc;
.abc(abc);

xyzzy

int foo;
.foo(foo);
INDATA

open my $in_fh1, '<', \$inData;

my $rx_pre    = qr{ [ .(] }xms;
my $rx_target = qr{  abc  }xms;

while (my $line = <$in_fh1>) {

    if ($line =~ m{ $rx_pre $rx_target }xms) {
        for my $i (1 .. 2) {
            (my $changed = $line) =~ s{ ($rx_pre $rx_target) }{${1}_$i
+}xmsg;
            print $changed;
            }
        }
    else {
        print $line;
        }

    }

^Z
int abc_1;
int abc_2;
.abc_1(abc_1);
.abc_2(abc_2);

xyzzy

int foo;
.foo(foo);
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