How can I combine hashes in Perl?

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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? (e.g. 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

Footnotes

1 * (aka associative-array, aka dictionary)

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