When I tried it (I don't use the debugger normally), I found that the warning is shown, if warnings are switched on. Here is a (slightly edited) session: 
anno@oliva> perl -wd -e ''

Loading DB routines from perl5db.pl version 1.28

[...]

  DB<1> $x = { 1 }
Odd number of elements in anonymous hash at (eval 10)[/usr/local/lib/p
+erl5/5.8.7/perl5db.pl:628] line 2.

[...]

[download]
So it seems the debugger has ways of restoring the warning state of the outer interpreter in an eval. (Can't be that hard). Why the original poster didn't see the warning remains an open question.

Anno

In reply to Re^3: warnings under the debugger by Anno
in thread warnings under the debugger by trwww

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