my %subs = (
    word1   => 'replace word1 with this text',
    word2   => 'replace word2 with this test',
);

while($document =~ s/\[(\w+)\]) {
    my $word = $1;
    my $rep = $subs{$word} || "!Unknown token: $word!";
    $document =~ s/\[$word\]/$subs{$word}/smg;
}