I'm so confused as to what you're going on about here. The code cited in the original post has absolutely nothing to do with "alternate delimiters for m//" and the various other things you're going on about.

All that is happening is that =~ will use ANY SCALAR on the right hand side as a regex. It stringifies it and treats that value as a regex, then applies it to the left hand as normal.

So in short, anything that produces a scalar can be used on the right hand side of the =~ operator, including, for example:

$a =~ { foo };
$b =~ [ "bar" ];
[download]

And so forth. The "m" isn't optional for these because they aren't regexes! They're simply scalar producers.