Perl testing patterns using Test2::V0, Test::More, prove runner, mocking, coverage with Devel::Cover, and TDD methodology. Use when writing Perl tests with Test2::V0 or Test::More, or measuring coverage.

Perl Testing Patterns Comprehensive testing strategies for Perl applications using Test2::V0, Test::More, prove, and TDD methodology. When to Activate Writing new Perl code (follow TDD: red, green, refactor) Designing test suites for Perl modules or applications Reviewing Perl test coverage Setting up Perl testing infrastructure Migrating tests from Test::More to Test2::V0 Debugging failing Perl tests TDD Workflow Always follow the RED GREEN REFACTOR cycle. Test::More Fundamentals The standard Perl testing module — widely used, ships with core. Basic Assertions SKIP and TODO Test2::V0 Modern Framework Test2::V0 is the modern replacement for Test::More — richer assertions, better diagnostics, and extensible. Why Test2? Superior deep comparison with hash/array builders Better diagnostic output on failures Subtests with cleaner scoping Extensible via Test2::Tools:: plugins Backward compatible with Test::More tests Deep Comparison with Builders Subtests Exception Testing with Test2 Test Organization and prove Directory Structure prove Commands .proverc Configuration Fixtures and Setup/Teardown Subtest Isolation Shared Test Helpers Place reusable helpers in t/lib/TestHelper.pm and load with use lib 't/lib' . Export factory functions like create test db() , create temp dir() , and fixture path() via Exporter . Mocking Test::MockModule For lightweight mock objects, use Test::MockObject to create injectable test doubles with mock() and verify calls with called ok() . Coverage with Devel::Cover Running Coverage Integration Testing Use in memory SQLite for database tests, mock HTTP::Tiny for API tests. Best Practices DO Follow TDD : Write tests before implementation (red green refactor) Use Test2::V0 : Modern assertions, better diagnostics Use subtests : Group related assertions, isolate state Mock external dependencies : Network, database, file system Use prove l : Always include lib/ in @INC Name tests clearly : 'user login with invalid password fails' Test edge cases : Empty strings, undef, zero, boundary values Aim for 80%+ coverage : Focus on business logic paths Keep tests fast : Mock I/O, use in memory databases DON'T Don't test implementation : Test behavior and output, not internals Don't share state between subtests : Each subtest should be independent Don't skip done testing : Ensures all planned tests ran Don't over mock : Mock boundaries only, not the code under test Don't use Test::More for new projects : Prefer Test2::V0 Don't ignore test failures : All tests must pass before merge Don't test CPAN modules : Trust libraries to work correctly Don't write brittle tests : Avoid over specific string matching Quick Reference Task Command / Pattern Run all tests prove lr t/ Run one test verbose prove lv t/unit/user.t Parallel test run prove lr j8 t/ Coverage report cover test && cover report html Test equality is($got, $expected, 'label') Deep comparison is($got, hash { field k = 'v'; etc() }, 'label') Test exception like(dies { ... }, qr/msg/, 'label') Test no exception ok(lives { ... }, 'label') Mock a method Test::MockModule new('Pkg') mock(m = sub { ... }) Skip tests SKIP: { skip 'reason', $count unless $cond; ... } TODO tests TODO: { local $TODO = 'reason'; ... } Common Pitfalls Forgetting done testing Missing l Flag Over Mocking Mock the dependency , not the code under test. If your test only verifies that a mock returns what you told it to, it tests nothing. Test Pollution Use my variables inside subtests — never our — to prevent state leaking between tests. Remember : Tests are your safety net. Keep them fast, focused, and independent. Use Test2::V0 for new projects, prove for running, and Devel::Cover for accountability.

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