We need to consider two aspects: How can we express this feature in our writeups? How could it work?

One way to express it might be some sort of minimal markup to toggle between normal and highlighted text. That markup would have to vanish in the downloaded source, of course (rule 1). It would be nice if it could be used anywhere within a line (rule 2). For the author or editor, it should not damage the readability of the source code too much (rule 3). Writeups written without this feature in mind should not change in appearance (rule 4).

To implement the feature, the PerlMonks engine would presumably need a new set of filters to render our highlightable code sections in the contexts that matter (viewing, editing, downloading). I won't go into details here, as I am just a happy user so far and ignorant of any internals.

As to a possible syntax, this could take us some consideration, but I have come up with a first draft. Please feel free to comment on either the request in general or the suggested syntax.

Unsurprisingly, maintaining source readability turns out to be a tough requirement. Perl has such a rich syntax by itself that hardly any in-line symbol can be added that would be clearly distinguishable as alien and at the same time not obscure anything.

• Highlightable code blocks need a new set of enclosing tags to avoid any ambiguity. I suggest <hcode>.
• Source lines are written as is, just as in ordinary code blocks.
• Tab characters should be avoided and replaced by the appropriate number of blanks.
• Our minimal markup -- turning highlighting on and off -- is achieved using extra lines.
• A line containing at least two but none other than ^ (caret) and . (dot) characters modifies the appearance of the previous line: Each character above a caret will be highlighted, all others not.
• A non-code line forces the next line to be treated as code, whatever ist is. That way a row of dots can be used to guard a similarly looking code line.

Example:

<hcode>
use strict;
use warnings;

print the_time(), "\n";

sub the_time {
    return scalar localtime @_;
...........^^^^^^
}

__END__
</hcode>
[download]

In this example, only the word scalar would be highlighted -- rendered in boldface, say.

With respect to rule 3, I found it easier to mentally filter out funny looking lines than funny looking embedded character sequences from Perl source. I prefer dots to underscores to let those lines appear more as a unit.

The second best solution I considered allowed only complete lines to be highlighted. It would require the whole code block to be indented and reserve the first column to certain control markers, somewhat like unified diff output.

Anyway, I think such a feature could be quite useful. I hope it will neither be too difficult to get implemented nor the final solution too awkward to become popular. Have fun!