This was the constructor:

sub new
{
    my ($class, %parameters) = \@_;

    my $self = bless ({}, ref ($class) || $class);

    return ($self);
}
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This came right out of ExtUtils::ModuleMaker. After reading some of the comments in this node, I flipped through the Camel and found this suggestion:

sub new 
{
    my $invocant = shift;
    my $class = ref($invocant) || $invocant;
    my $self = { };
    bless($self, $class);
    return $self;
}
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Which appears to me to do basically the same thing (except it's not taking a hashref as a param, which shouldn't matter, and I don't need it anyway). However, the tests all pass with this version.

I see what is going on now. This line:

my ($class, %parameters) = \@_;
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makes a reference to the array @_, and assigns $class that reference. %parameters is left blank. Calling ref on an unblessed array reference returns ARRAY; the next line then blesses $self as ARRAY. Removing the backslash in the first line would cause $class to get the first argument, and %parameters to get the other arguments converted into a hash, which seems to be more what you want.