You shouldn't do such tests in different lines of the debugger but in one line.

  DB<137> $!=undef; readdir $x or print 0+$!," ",$!
9 Bad file descriptor
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a) Each line is evaled in a different context!

b) The debugger saves and restores special variables between evals, cause otherwise evaled code would change the error-state of the debugged program.

Thanks BTW for discovering this!

It shows fundamental differences between debugger and REPL functionality!

UPDATE

seems that I was wrong, the debugger can handle ERRNO in different lines, it's '$!=undef' which does the difference!

  DB<100> opendir $x,'.';closedir $x;

  DB<101> readdir $x 

  DB<102> print 0+$!," ",$!
29 Illegal seek

  DB<103> $!=undef

  DB<104> readdir $x 

  DB<105> print 0+$!," ",$!
9 Bad file descriptor
[download]

For more details see Re: Testing for readdir failure (Perl-bug)