A Perl REPL would be very nice, agreed, but I do not think that that would break much code, especially if input-dependent: present REPL if STDIN is a terminal, read program if STDIN is a pipe or file.
I do not know if retiring XS is as good an idea as it sounds — XS contributes significantly to perl's performance with some extensions.
I am not sure that there are any usable sigil characters left in ASCII, although caret might be usable if it can be disambiguated from bitwise-XOR. Ampersand is both code sigil and bitwise-AND, so this might be workable.
Generating webassembly sounds like a role for a B:: module.
Snippets of code should be wrapped in
<code> tags not<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
intervention).