Testing · Perl
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How to add tests to a legacy Perl application.
Add useful tests to legacy Perl without rewriting it first: characterize behaviour, create seams, protect risky paths and grow coverage where change happens.
Maple · Mecha · Perl
01
Begin at a boundary you can observe
Choose a request, command, file transformation, database operation or other behaviour whose inputs and outputs can be captured reliably.
02
Write characterization tests
A characterization test records what the system currently does, including behaviour you may later decide to change. Its first job is to warn you when modernization changes something unexpectedly.
03
Create narrow seams
Move small pieces of logic behind module or function boundaries where dependencies can be controlled. Avoid a giant refactor whose success depends on all new tests passing at once.
04
Test risk, not line count
Prioritize authentication, money, destructive operations, complex transformations, historically fragile paths and code you are about to modify.
05
Grow tests alongside maintenance
Every repaired bug and every changed feature is an opportunity to leave a stronger safety net in the area that has proven it needs one.
06
A practical next step
Choose the next change you already need to make. Write a test that demonstrates the current behaviour first, then use that test to protect the repair.
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