A little reordering of module declarations fixes it:

use strict;
use warnings;

package Test;

sub new {
        my $proto = shift;
        my $class = ref($proto) || $proto;
        my $self = {};
        return bless $self, $class;
}

package Test1;
@Test1::ISA = qw(Test);

package Main;

my $test1 = new Test1;
print "isa ",ref($test1),"\n"
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