Jeffrey Friedl talks a bit about this in Mastering Regular Expressions in the section called "A Warning About Embedded Code and my Variables" (page 338-339). His conclusion on the matter is that an embedded code construct is in fact a closure.

This means that using a lexical variable inside an embedded code construct in a regular expression binds the instance of the lexical variable in existence at the moment the regex is compiled to the regex. As far as I understand, this means that:

my $regex = qr /(              # Start capture
                \(              # Start with '(',
                (?:             # Followed by
                (?>[^()]+)      # Non-parenthesis
                |(??{ $regex }) # Or a balanced () block
                )*              # zero or more times
                \)              # Close capture
                )/x;            # Ending with ')'
[download]

with regard to liz' remark about lexicals at compile time, is "interpreted" (pardon the hand-waving) as:

my $regex = qr /(              # Start capture
                \(              # Start with '(',
                (?:             # Followed by
                (?>[^()]+)      # Non-parenthesis
                |(??{ undef })  # <-- Note! 'or undef'
                )*              # zero or more times
                \)              # Close capture
                )/x;            # Ending with ')'
[download]

which will match the innermost parentheses. We don't have any undefs in the target string, so how can this part of the construct match?

Trying to write this down in a sensible manner proved to be quite a challenge, so I apologize if the preceeding section is hard to understand. But to quote Friedl from the aforementioned section: "Warning: this section is not light reading." :o)

Hope this helps.

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