I see two holes in your argument.
1) That particular string is non-numeric and it does get converted to numeric. What would be the point of not issuing a "non-numeric isn't numeric" warning when it happens because the operator works with some other strings?
2) perl -wle "$s='abc'; $s--; print $s" doesn't give a warning either, yet non-numeric strings are never legal for the post-decrement operator.
I suspect the warning is surpressed as a side-effect (intentional or not) of supressing the undefined warning when executing my $i; $i++;.