I'm not the best one to answer questions about the Perl debugger, but I can answer some more general questions.

Q0) ... we begin the game with a string that has 2 backslashes ... the output differs from the original string by having one fewer backslash.
Due to the way the single-quote string constructor handles backslashes (escapes), the \\ will in this case compile to a single literal backslash. See Quote and Quote-like Operators and the discussion of q/STRING/ in Quote-Like Operators.
Q1) What makes you think this is a left-curly brace?
I'm not quite sure what "this" refers to, but do you dispute that there is a left-curly (and a right-curly) in the \x{A3f4} string? What else would you call it/them?
Q2: What is \x{value} called? ...
I would call it (or in this case \x{A3f4}) "the string compiled from '\\x{A3f4}'". The \x part has nothing to do with the /x or /xx regex modifiers.

Give a man a fish:  <%-{-{-{-<

In reply to Re^5: Unescaped left brace in regex is passed through in regex by AnomalousMonk
in thread Unescaped left brace in regex is passed through in regex by gzh

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