FAQ 4.67 Why does passing a subroutine an undefined element in a hash create it?

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4.67: Why does passing a subroutine an undefined element in a hash create it?

If you say something like:

somefunc($hash{"nonesuch key here"});

Then that element "autovivifies"; that is, it springs into existence
whether you store something there or not. That's because functions get
scalars passed in by reference. If somefunc() modifies $_[0], it has to
be ready to write it back into the caller's version.

This has been fixed as of Perl5.004.

Normally, merely accessing a key's value for a nonexistent key does
*not* cause that key to be forever there. This is different than awk's
behavior.

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