Although I don't like to rely on autovivication because it doesn't work for every case. I don't like to follow the boundary of arbitrary rules (unless avoiding tax ;);)) So in my code, for example, you'll find initialisations ahead of the array reference usage like:
$bysource{$source} ||= [];
push etc.

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In reply to Re: help understanding code snippet by anonymized user 468275
in thread help understanding code snippet by Anonymous Monk

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