Not an answer, but I wonder why you do

sub _interval {
    my ($min, $max) = ( min(@_), max(@_) );
    return bless [ ... ], __PACKAGE__;
}

Math::Interval::_interval(...)
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instead of

sub new {
    my $class = shift;
    my ($min, $max) = ( min(@_), max(@_) );
    return bless [ ... ], $class;
}

Math::Interval->new(...)
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