However, Perl is not psychic. So, when you see most constructors ending with:   bless $self, $class; $self is not modified.

This is wrong, $self is modified. Try e.g. this:

sub foo {
    my $self = {};
    bless $self, 'Bar';
    return $self;
}

print "isa Bar.\n" if foo()->isa('Bar');
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