How can I combine hashes in Perl?

Question

What is the best way to combine both hashes into %hash1? I always know that %hash2 and %hash1 always have unique keys. I would also prefer a single line of code if possible.

$hash1{'1'} = 'red';
$hash1{'2'} = 'blue';
$hash2{'3'} = 'green';
$hash2{'4'} = 'yellow';

Answer 1

Quick Answer (TL;DR)

    %hash1 = (%hash1, %hash2)

    ## or else ...

    @hash1{keys %hash2} = values %hash2;

    ## or with references ...

    $hash_ref1 = { %$hash_ref1, %$hash_ref2 };

Overview

• Context: Perl 5.x
• Problem: The user wishes to merge two hashes¹ into a single variable

Solution

• use the syntax above for simple variables
• use Hash::Merge for complex nested variables

Pitfalls

• What do to when both hashes contain one or more duplicate keys
  • (see e.g., Perl - Merge hash containing duplicate keys)
  • (see e.g, Perl hashes: how to deal with duplicate keys and get possible pair)
• Should a key-value pair with an empty value ever overwrite a key-value pair with a non-empty value?
  • What constitutes an empty vs non-empty value in the first place? (eg. undef, zero, empty string, false, falsy ...)

See also

• PM post on merging hashes
• PM Categorical Q&A hash union
• Perl Cookbook 5.10. Merging Hashes
• websearch://perlfaq "merge two hashes"
• websearch://perl merge hash
• https://metacpan.org/pod/Hash::Merge

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Footnotes

¹ * (aka associative-array, aka dictionary)