Clunky, but may be instructional.

while (m(<code>|\[|<!--)g) {
    if ($& eq '<code>') {
        s{\G(.+?)</ code>}{$GREEN$1$RESET};
        pos($_) += length("$GREEN$1$RESET");
    }
    elsif ($& eq '[') {
        s{\G(.+?)\]}{$RED$1$RESET};
        pos($_) += length("$RED$1$RESET");
    }
    elsif ($& eq '<!--') {
        s{\G(.+?)-->}{$YELLOW<!--$1$-->RESET};
        pos($_) += length("$YELLOW$1$RESET");
    }
}
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