Is that documented behaviour? Where?

Yes, in perlre, as follows:

An unescaped "$" or "@" interpolates the corresponding variable, while escaping will cause the literal string "\$" to be matched.

(Though in the version of the perlre man page I have installed, for perl 5.8.8, this sentence comes second in a paragraph that begins with:

You cannot include a literal "$" or "@" within a "\Q" sequence."

I can understand that some might consider this obscure.)

In reply to Re^2: Regex help by graff
in thread Regex help by Anonymous Monk

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