How to use List::MoreUtils uniq to remove duplicates in Perl?

Removing Duplicates from an Array Using List::MoreUtils::uniq

The List::MoreUtils module provides several useful utilities for list manipulation, including the uniq function which can be used to remove duplicate elements from an array. It offers a simplified and efficient way to get unique values while preserving the order of their first occurrence.

What is uniq in List::MoreUtils?

uniq takes a list and returns a new list containing only the first occurrence of each distinct element, effectively removing duplicates. Unlike uniq in some other modules, it preserves the original order of elements.

Basic Usage

• Import uniq from List::MoreUtils.
• Pass your list (array) to uniq.
• Assign the returned list to a new array or overwrite the existing one.

This requires you have List::MoreUtils installed. It is a core module bundled with Perl since 5.10, so you likely have it already.

Example Code

use strict;
use warnings;

# Import uniq from List::MoreUtils (core module since Perl 5.10)
use List::MoreUtils qw(uniq);

# Example array with duplicates
my @data = qw(apple banana apple orange banana grape apple);

# Remove duplicates, preserving first occurrence order
my @unique = uniq @data;

print "Original list: @data\n";
print "Unique list: @unique\n";

Explanation

• qw() is a convenient way to create a list of strings.
• uniq @data processes the list and returns only unique values.
• The result is stored in @unique and printed out.

Output

When you run the above Perl script, you'll get:

Original list: apple banana apple orange banana grape apple
Unique list: apple banana orange grape

Perl Concepts Demonstrated

• Sigils: @ sigil denotes arrays (@data, @unique).
• Context: uniq expects a list, returns a list, so scalar vs. list context matters.
• Modules and Importing: You import uniq explicitly to use it. This is common CPAN practice.

Common Pitfalls

• Make sure to import uniq properly or you’ll get an error.
• List::MoreUtils::uniq removes duplicates based on string equality, not deeper data structures or references.
• Preserved order is based on first occurrence only — last occurrences are discarded.
• Don’t confuse List::MoreUtils::uniq with List::Util::uniq, which appeared later with different behavior in Perl 5.36+.

Summary

Using List::MoreUtils::uniq is a clean, simple way to remove duplicates from an array in Perl while preserving order. It’s widely available on modern Perl installations and is perfect for beginners learning list manipulation.