Arguments to subs are lvalues, and lvalues autovivify. Feature.

Incidentally, you can also induce lvalue context by taking a reference. Interestingly, this program generates a warning about Useless use of reference constructor, but without taking the reference, it dies on the undefined value. 

use warnings;
use strict;

my $x;
\@$x;
print @$x;

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It should go without saying that using these side-effects to initialize is not a good programming practice.
Caution: Contents may have been coded under pressure.

In reply to Re: Autovivification of scalars in sub calls by Roy Johnson
in thread Autovivification of scalars in sub calls by tlm

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