please note the single quotes, it creates just the string \x{A3f4}

only two characters need to be escaped inside single-quotes ... the single-quote and consequently the escape-character, which is the backslash.

"\x{HEX}" is a notation for hex-character only inside double-quotes, so it seems someone is trying to parse code similar to Perl.

I ran the debugger with 5.32 without warnings pre-activated, I'm surprised about your errors. Looks like you ran something like perl -dwe0

And you are seeing the internal stack-trace of the debugger, I don't wanna comment on the perldebguts

The debugger is mostly written in Perl, but using hooks written in C.

I.O.W. perl5db.pl is written in Perl, but only possible because of the debugging API written in C, expecting to find functions like &DB::DB

And there are some old debugging flags available written in C, things I've never used.

Greetings from Krautland ;-)