That's allowed and quite fine, in my opinion. Another approach is to use a reference, as you say:

sub register_plugin {
    my $class = shift;
    if (defined(my $constructor = UNIVERSAL::can($class, 'new'))) {
        push @callbacks, sub { $constructor->($class) };
    }
}

foreach my $callback (@callbacks) {
    my $obj = $callback->();
    $obj->hello();
}
[download]

It's a little more paranoid, but I prefer your original version.