The eval block appears to fail even on a system that does have Test::Warn.

Erm.. that's wierd. Are you sure you're calling Test::Warn->import(); instead of Test::Warn::import() ?

Oops. You're right. I did have Test::Warn::import.

I fixed this typo, but all the same, I get the same error as before and the test script dies.

$ perl t/01-test_errors.t 
1..43
$@: 
ARRAY(0x6acb08) is not a hash reference.
ok 1 - non-hash reference to cpf2data()
Can't call method "warning_like" without a package or object reference at t/01-test_errors.t line 18.
# Looks like you planned 43 tests but only ran 1.
# Looks like your test died just after 1.

Here, ARRAY... is the warning that I want to trap.

Looks like you're not skipping the tests if Test::Warn isn't installed (i mean, warning_like is a Test::Warn function and should be skipped if loading Test::Warn doesn't succeed)

update:

Note you (probably) should use skip blocks to skip tests in Test::More:

SKIP: {
  skip "For some reason",$number_of_tests;
  # tests to skip here
}
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