/ doesn't have a meaning in regex, but it's not an alpha character, so
if you want to "match literally" it "must be quoted" as the first error
message tells you.
On top of that, balanced characters like () and {} aren't allowed for
the s/// form (only for the assignment form which looks like s{ foo } =
"bar").
Sadly, I couldn't find any docs about the assignment form of s///, but
it does explain the thing with balanced parenthesis:
https://docs.perl6.org/syntax/s$SOLIDUS%20$SOLIDUS%20$SOLIDUS
Hope that helps!
- Timo
