print do_stuff();

##</code><code>##


use strict;
use warnings;
use 5.012;

my $target = 'hello';
my %should_replace = ('hello' => 1);
my $repl = 'Hello';


$_ = 'hello world goodbye';

s/$target/print $should_replace{$target} ? "$repl " : "[$repl] "/ex;

##</code><code>##

use strict;
use warnings;
use 5.012;

my $text = 'hello world goodbye';
my $target = 'hello';
my %should_replace = ('hello' => undef);
my $repl = 'Hello';

if ($text =~ $target) {
    my $output = exists $should_replace{$target} ? "$repl " : "[$repl] ";
    say $output;
}

##</code><code>##

use strict;
use warnings;
use 5.012;

my $text = 'hello world goodbye';
my $target = 'hello';
my %keys = ('world' => undef);
my $repl = 'Hello';


if ($text =~ $target) {
    my $output;

    if (exists $keys{$target}) {
        $output =  $repl; 
    }
    else {
        $output = "[$repl]";
    }

    say "$output ";

}


--output:--
[Hello]