Deprecated left curly bracket in perl regex - exactly when?

perldoc perlre says this

(If a curly bracket occurs in any other context and does not form part of a backslashed sequence like \x{...} , it is treated as a regular character. However, a deprecation warning is raised for all such occurrences, and in Perl v5.26, literal uses of a curly bracket will be required to be escaped, say by preceding them with a backslash ("{" ) or enclosing them within square brackets ("[{]" ). This change will allow for future syntax extensions (like making the lower bound of a quantifier optional), and better error checking of quantifiers.)

OK, so the following prints the deprecation message.

perl -lE 'm/x{x}/'

why the following isn't?

perl -lE 'm/x({x})/'

e.g. in the capture group is the { allowed unescaped? Probably not because the

perl -lE 'm/x(x{x})/'

also prints the warning.

So, what is the exact "logic"?

Ps: i will escape every literal {, but wondering about the rationale behind the above.