You are numbering all abc's. There is no need to match surrounding characters. Note that the way I have rewritten your test cases removes all ambiguity concerning white space. 
use strict;
use warnings;
use Test::More;

my @cases = (
  # Input            Expected                             Description
  ["int abc;\n",    "int abc_1;\n\nint abc_2;\n\n",         'line 1'],
  [".abc(abc);\n",  ".abc_1(abc_1);\n\n.abc_2(abc_2);\n\n", 'line 2'],
);

plan tests => scalar @cases;

foreach my $case (@cases) {
    my $line = $case->[0];
    my $out = '';
    $out .= ($line =~ s/abc/abc_$_/gr) . "\n" for (1,2);
    is( $out, $case->[1], $case->[2] );
}

[download]
OUTPUT:
1..2
ok 1 - line 1
ok 2 - line 2

[download]
Bill

In reply to Re^5: Please correct me in the above regular expression by BillKSmith
in thread Please correct me in the above regular expression by suvendra123

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