ah, thank you!

I missed there are warnings while testing in the debugger¹!

However this gives me the possibility to change $SIG{__WARN__} to deal with this kind of errors...e.g. by dieing.

1)... hmm but why?

UPDATE:

OK starting the debugger with -w switch helps.

:~$ perl -w -de0
...
  DB<1>  $x="6gg"
 
  DB<2> p -$x
Argument "6gg" isn't numeric in negation (-) at (eval 6)[/usr/share/pe
+rl/5.10/perl5db.pl:638] line 2.
...
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