Something like this, perhaps:

#!/usr/bin/perl
use strict;
use warnings;

my %replace = (
    Not         => 'Just',
    Stupid      => 'Perl',
);

my $string = "[Not] Another [Stupid] Hacker";

$string =~ s/\[([^\]]+)\]/$replace{$1}/g;

print $string, $/;

__OUTPUT__
Just Another Perl Hacker
[download]