Testing in Perl

Introduction

A script you cannot verify automatically is a script you will eventually break without noticing. Perl has a mature, built-in testing culture centered on the core module Test::More, and a standard test runner called prove. This lesson writes a small module and a real test file for it.

Why Test Perl Code

Tests let you change code with confidence - if you break something, a test fails immediately instead of the bug surfacing weeks later in production. They also work as living documentation: a well-named test describes exactly what a function is supposed to do.

Introducing Test::More

Test::More has shipped with Perl since version 5.6.2, so it is available essentially everywhere without installing anything extra. It provides a family of small, focused assertion functions, each of which prints a standardized "ok" or "not ok" line that testing tools can parse.

The ok Function

ok($condition, $description) is the simplest assertion: it passes if $condition is true, and fails otherwise. The description shows up in the test output, so make it specific enough to be useful on its own.

ok(1 == 1, 'one equals one');
ok(defined $some_value, 'the value is defined');

The is Function for Comparisons

is($got, $expected, $description) compares two values with eq semantics and, importantly, prints both the value you got and the value you expected when the test fails - far more useful for debugging than a plain ok(...) would be.

is(2 + 2, 4, '2 + 2 equals 4');
is('perl' . '!', 'perl!', 'string concatenation works');

Testing Errors with like

like($got, qr/pattern/, $description) checks that a value matches a regular expression - the standard way to test that die produced the error message you expected, without requiring an exact string match.

Declaring the Test Plan

Test::More can either be told up front exactly how many tests to expect (use Test::More tests => 4;) or, more commonly in modern code, you simply call done_testing() once at the very end of the file. Either approach lets the test harness detect if the script died partway through and ran fewer tests than intended.

A Complete Test File Example

Here is a small module and a matching test file that exercises it, using ok, is, and like together.

package Calculator;
use strict;
use warnings;

sub add {
    my ($a, $b) = @_;
    return $a + $b;
}

sub divide {
    my ($a, $b) = @_;
    die "Cannot divide by zero\n" if $b == 0;
    return $a / $b;
}

1;

use strict;
use warnings;
use Test::More;
use Calculator;

is(Calculator::add(2, 3), 5, 'add() sums two numbers');
is(Calculator::add(-1, 1), 0, 'add() handles negatives');
is(Calculator::divide(10, 2), 5, 'divide() computes correctly');

eval { Calculator::divide(10, 0) };
like($@, qr/Cannot divide by zero/, 'divide() dies on zero');

done_testing();

Running Tests with prove

prove is the standard command-line test harness that ships with Perl. Point it at one or more .t files (by convention kept in a t/ directory) and it runs them, summarizing pass/fail results.

prove -v t/calculator.t

Common Mistakes

Best Practices

Frequently Asked Questions

Key Takeaways

Summary

Test::More and prove give Perl a lightweight, built-in testing workflow with no extra installation required. With testing covered, the next lesson turns to something every language has plenty of: the mistakes beginners make most often.