diff --git a/action.yml b/action.yml
index c6ff610..223e1cc 100644
--- a/action.yml
+++ b/action.yml
@@ -34,4 +34,4 @@ outputs:
 
 runs:
   using: 'node20'
-  main: 'dist/main.js'
+  main: 'dist/action.js'
diff --git a/dist/main.js b/dist/action.js
similarity index 99%
rename from dist/main.js
rename to dist/action.js
index 157f06b..16f4b38 100644
--- a/dist/main.js
+++ b/dist/action.js
@@ -18862,7 +18862,7 @@ var require_core = __commonJS({
       process.env["PATH"] = `${inputPath}${path.delimiter}${process.env["PATH"]}`;
     }
     exports2.addPath = addPath;
-    function getInput2(name, options) {
+    function getInput(name, options) {
       const val = process.env[`INPUT_${name.replace(/ /g, "_").toUpperCase()}`] || "";
       if (options && options.required && !val) {
         throw new Error(`Input required and not supplied: ${name}`);
@@ -18872,9 +18872,9 @@ var require_core = __commonJS({
       }
       return val.trim();
     }
-    exports2.getInput = getInput2;
+    exports2.getInput = getInput;
     function getMultilineInput(name, options) {
-      const inputs = getInput2(name, options).split("\n").filter((x) => x !== "");
+      const inputs = getInput(name, options).split("\n").filter((x) => x !== "");
       if (options && options.trimWhitespace === false) {
         return inputs;
       }
@@ -18884,7 +18884,7 @@ var require_core = __commonJS({
     function getBooleanInput(name, options) {
       const trueValue = ["true", "True", "TRUE"];
       const falseValue = ["false", "False", "FALSE"];
-      const val = getInput2(name, options);
+      const val = getInput(name, options);
       if (trueValue.includes(val))
         return true;
       if (falseValue.includes(val))
@@ -18893,7 +18893,7 @@ var require_core = __commonJS({
 Support boolean input list: \`true | True | TRUE | false | False | FALSE\``);
     }
     exports2.getBooleanInput = getBooleanInput;
-    function setOutput2(name, value) {
+    function setOutput(name, value) {
       const filePath = process.env["GITHUB_OUTPUT"] || "";
       if (filePath) {
         return file_command_1.issueFileCommand("OUTPUT", file_command_1.prepareKeyValueMessage(name, value));
@@ -18901,16 +18901,16 @@ Support boolean input list: \`true | True | TRUE | false | False | FALSE\``);
       process.stdout.write(os.EOL);
       command_1.issueCommand("set-output", { name }, utils_1.toCommandValue(value));
     }
-    exports2.setOutput = setOutput2;
+    exports2.setOutput = setOutput;
     function setCommandEcho(enabled) {
       command_1.issue("echo", enabled ? "on" : "off");
     }
     exports2.setCommandEcho = setCommandEcho;
-    function setFailed2(message) {
+    function setFailed(message) {
       process.exitCode = ExitCode.Failure;
       error(message);
     }
-    exports2.setFailed = setFailed2;
+    exports2.setFailed = setFailed;
     function isDebug() {
       return process.env["RUNNER_DEBUG"] === "1";
     }
@@ -18996,7 +18996,7 @@ Support boolean input list: \`true | True | TRUE | false | False | FALSE\``);
 });
 
 // src/main.mjs
-var core2 = __toESM(require_core(), 1);
+var import_core = __toESM(require_core(), 1);
 
 // src/get-prereqs.mjs
 var import_promises = __toESM(require("node:fs/promises"), 1);
@@ -25394,10 +25394,14 @@ var getPrereqs = async ({
 
 // src/main.mjs
 var run = async () => {
-  const phases = core2.getInput("phases").split(/\s+/);
-  const relationships = core2.getInput("relationships").split(/\s+/);
-  const features = core2.getInput("features").split(/\s+/);
-  const sources = core2.getInput("sources").split(/\s+/);
+  const phasesInput = import_core.default.getInput("phases");
+  const relationshipsInput = import_core.default.getInput("relationships");
+  const featuresInput = import_core.default.getInput("features");
+  const sourcesInput = import_core.default.getInput("sources");
+  const phases = new Set(phasesInput.split(/\s+/));
+  const relationships = new Set(relationshipsInput.split(/\s+/));
+  const features = new Set(featuresInput.split(/\s+/));
+  const sources = sourcesInput.split(/\s+/);
   const { perl, ...prereqs } = await getPrereqs({
     phases,
     relationships,
@@ -25405,23 +25409,26 @@ var run = async () => {
     sources
   });
   if (perl) {
-    core2.setOutput("perl", dottedVersion(perl));
+    import_core.default.setOutput("perl", dottedVersion(perl));
   }
   const prereqString = Object.keys(prereqs).map((module2) => `${module2}
 `).join("");
   const prereqVersionString = Object.entries(prereqs).map(([module2, version2]) => `${module2}${cpanmVersion(version2)}
 `).join("");
-  core2.setOutput("prereqs", prereqVersionString);
-  core2.setOutput("prereqs-no-version", prereqString);
-  core2.setOutput("prereqsJSON", JSON.stringify(prereqs));
+  import_core.default.setOutput("prereqs", prereqVersionString);
+  import_core.default.setOutput("prereqs-no-version", prereqString);
+  import_core.default.setOutput("prereqsJSON", JSON.stringify(prereqs));
 };
-(async () => {
+var main_default = async () => {
   try {
     await run();
   } catch (error) {
-    core2.setFailed(error.message);
+    import_core.default.setFailed(error.message);
   }
-})();
+};
+
+// src/action.mjs
+main_default();
 /*! Bundled license information:
 
 undici/lib/fetch/body.js:
diff --git a/package-lock.json b/package-lock.json
index 71ff3b0..130aace 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -19,6 +19,7 @@
         "esbuild": "^0.23.1",
         "eslint": "^9.9.1",
         "eslint-plugin-mocha": "^10.5.0",
+        "esmock": "^2.6.7",
         "globals": "^15.9.0",
         "mocha": "^10.7.3",
         "peggy": "^4.0.3"
@@ -1548,6 +1549,16 @@
         "node": "*"
       }
     },
+    "node_modules/esmock": {
+      "version": "2.6.7",
+      "resolved": "https://registry.npmjs.org/esmock/-/esmock-2.6.7.tgz",
+      "integrity": "sha512-4DmjZ0qQIG+NQV1njHvWrua/cZEuJq56A3pSELT2BjNuol1aads7BluofCbLErdO41Ic1XCd2UMepVLpjL64YQ==",
+      "dev": true,
+      "license": "ISC",
+      "engines": {
+        "node": ">=14.16.0"
+      }
+    },
     "node_modules/espree": {
       "version": "10.1.0",
       "resolved": "https://registry.npmjs.org/espree/-/espree-10.1.0.tgz",
diff --git a/package.json b/package.json
index 38f76a5..61fc9fa 100644
--- a/package.json
+++ b/package.json
@@ -10,7 +10,7 @@
     "build-peg": "npm run build-peg:ini && npm run build-peg:cpanfile",
     "build-peg:cpanfile": "peggy src/parser/cpanfile-peg.pegjs --format es -o src/parser/cpanfile-peg.mjs",
     "build-peg:ini": "peggy src/parser/ini-peg.pegjs --format es -o src/parser/ini-peg.mjs",
-    "build": "npm run build-peg && esbuild src/main.mjs --outdir=dist/ --bundle --platform=node --target=node20"
+    "build": "npm run build-peg && esbuild src/action.mjs --outdir=dist/ --bundle --platform=node --target=node20"
   },
   "repository": {
     "type": "git",
@@ -34,6 +34,7 @@
     "esbuild": "^0.23.1",
     "eslint": "^9.9.1",
     "eslint-plugin-mocha": "^10.5.0",
+    "esmock": "^2.6.7",
     "globals": "^15.9.0",
     "mocha": "^10.7.3",
     "peggy": "^4.0.3"
diff --git a/src/action.mjs b/src/action.mjs
new file mode 100644
index 0000000..6c4a38e
--- /dev/null
+++ b/src/action.mjs
@@ -0,0 +1,3 @@
+import main from './main.mjs';
+
+main();
diff --git a/src/main.mjs b/src/main.mjs
index ca2ca27..1e78e29 100644
--- a/src/main.mjs
+++ b/src/main.mjs
@@ -1,12 +1,17 @@
-import * as core from '@actions/core';
+import core from '@actions/core';
 import { getPrereqs } from './get-prereqs.mjs';
 import { cpanmVersion, dottedVersion } from './cpan-versions.mjs';
 
-const run = async () => {
-  const phases = core.getInput('phases').split(/\s+/);
-  const relationships = core.getInput('relationships').split(/\s+/);
-  const features = core.getInput('features').split(/\s+/);
-  const sources = core.getInput('sources').split(/\s+/);
+export const run = async () => {
+  const phasesInput = core.getInput('phases');
+  const relationshipsInput = core.getInput('relationships');
+  const featuresInput = core.getInput('features');
+  const sourcesInput = core.getInput('sources');
+
+  const phases = new Set(phasesInput.split(/\s+/));
+  const relationships = new Set(relationshipsInput.split(/\s+/));
+  const features = new Set(featuresInput.split(/\s+/));
+  const sources = sourcesInput.split(/\s+/);
 
   const { perl, ...prereqs } = await getPrereqs({
     phases,
@@ -31,11 +36,11 @@ const run = async () => {
   core.setOutput('prereqsJSON', JSON.stringify(prereqs));
 };
 
-(async () => {
+export default async () => {
   try {
     await run();
   }
   catch (error) {
     core.setFailed(error.message);
   }
-})();
+};
diff --git a/test/action.test.mjs b/test/action.test.mjs
new file mode 100644
index 0000000..79dfc05
--- /dev/null
+++ b/test/action.test.mjs
@@ -0,0 +1,53 @@
+import { describe, it } from 'mocha';
+import { expect } from 'chai';
+
+import { dirname, join as joinPath } from 'node:path';
+import { fileURLToPath } from 'url';
+
+import esmock from 'esmock';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+
+const mockAction = async (module) => {
+  let outputs;
+  let inputs;
+  let failed;
+  const main = await esmock(module, {
+    '@actions/core': {
+      getInput:          key => (inputs[key] || '').trim(),
+      getMultilineInput: key => (inputs[key] || '').split(/\n/).map(t => t.trim()),
+      setOutput:         (key, value) => { outputs[key] = value; },
+      setFailed:         (message) => { failed = message; },
+    },
+  });
+  return async (input) => {
+    outputs = {};
+    inputs = input;
+    failed = undefined;
+    await main();
+    if (failed) {
+      throw new Error(failed);
+    }
+    return {
+      outputs,
+    };
+  };
+};
+
+const main = await mockAction('../src/main.mjs');
+
+describe('GitHub action', function () {
+  it('runs', async function () {
+    const { outputs } = await main({
+      sources:       joinPath(__dirname, 'corpus', 'META.json'),
+      phases:        'test',
+      relationships: 'recommends',
+      features:      '',
+    });
+    expect(outputs).to.be.deep.equal({
+      'prereqs':            'CPAN::Meta~2.120900\n',
+      'prereqs-no-version': 'CPAN::Meta\n',
+      'prereqsJSON':        '{"CPAN::Meta":">=2.120900"}',
+    });
+  });
+});