':Constructor' is not a valid Perl attribute,
Just replace those attributes by the word ':method' and your code will run fine.

package Attr;

sub new :method { return bless {}, shift }

sub set_blah :method {
    my ( $self, $blah ) = @_;
    $self->{blah} = $blah;
    return $self;
}

sub get_blah :method {
    my $self = shift;
    return $self->{blah};
}

1;

package main;

my $obj = Attr->new;
$obj->set_blah('I just set something!');
print $obj->get_blah;
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