I am not sure about that.

It is perfectly allowed to use a character class Perl short-cut within brackets

I get the same results, but I wonder where that is documented ...

(Some doc searching and a short meeting later ...)

It's hidden in perlrecharclass:

You can put any backslash sequence character class (with the exception of \N and \R) inside a bracketed character class, and it will act just as if you had put all characters matched by the backslash sequence inside the character class. For instance, [a-f\d] matches any decimal digit, or any of the lowercase letters between 'a' and 'f' inclusive.

[...]

Examples:

/[\p{Thai}\d]/     # Matches a character that is either a Thai
                   # character, or a digit.
/[^\p{Arabic}()]/  # Matches a character that is neither an Arabic
                   # character, nor a parenthesis.
[download]

So I stand corrected.

[\d] means the same as [0-9] [\w] would mean same as [a-zA-Z0-9_]

Well, that's only true if you ignore Unicode. perlre points to perlunicode, which has this nice short explaination:

\p{Word}

This is the same as \w, including over 100_000 characters beyond ASCII.

Alexander