Sorry, I forgot that you'll need to call the constructor of the class you're blessing. Here's how it should be:
sub new {
return bless Gtk3::SourceView::CompletionProvider->new();
}
Here's an example of some code from my gtk2-perl project, SeekMIDI at
https://github.com/oldtechaa/seekmidi/:
use Gtk2;
use base 'Gtk2::VBox';
sub new {
my $topVBox = bless Gtk2::VBox->new();
return($topVBox);
}