Searches a string for a pattern, and if found, replaces that pattern with the replacement text and returns the number of substitutions made. Otherwise it returns false (specifically, the empty string).

All function calls in your code are replaced by their return values. Your code is essentially this:

print do_stuff();
[download]

When perl encounters the function call do_stuff(), perl halts execution of the print statement, then perl executes the function do_stuff(), then perl replaces the call to do_stuff() in your code with do_stuff()'s return value.

You could do this:

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;
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...but that's pretty ugly. I think the following are better alternatives:

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;
}
[download]

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]
[download]

The goal is code clarity--not cramming as much code into one line as possible.

Is there a reason you aren't putting the replacement text in the hash?