Are you doing this in a test suite as the .t suggests? Then use Test::More:

use warnings;
use strict;
use Test::More qw(no_plan);

my $foo = {};
isa_ok($foo, 'My::Object');
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which gives:

not ok 1 - The object isa My::Object
#     Failed test (temp.pl at line 6)
#     The object isn't a 'My::Object' it's a 'HASH'
1..1
# Looks like you failed 1 test of 1.
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