See How to ask better questions using Test::More and sample data

use strict;
use warnings;

use Test::More tests => 2;                      

my $abc  = 'dgrs';
my $n    = 1;
my $want = 'dgrs_1';
my $have = '';

$have = $abc . '_' . $n;
is $have, $want, 'Concatenation operator (perldoc perlintro)';

$have = "${abc}_$n";
is $have, $want, 'String interpolation (perldoc perlop)';
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I also strongly recommend that you pick an indentation scheme and stick to it. Random whitespace is doing you no favours.