You are right that common usage in popular OO languages leans that way. As I stated I tend have that expectation myself. However:

Can an abstract class not have a constructor?

Should a subtype be able to be substituted for a supertype in any situation?

Should I be concerning myself with my object's concrete type on the typical constructor call?

When I consider these questions I wonder about the validity of my initial expectation.

It depends on what the meaning of the word 'isa' is.

Regarding:

my $frob = new Frob();
[download]

I have a greater expectation, in Perl, that Frob() is a routine call returning something here. That bothers me. This seems a very queer, obstrusive, distracting use of extraneous parentheses.