Its called autovivification, it means if you use it like an array ref it magically becomes an arrayref

use Data::Dump::Streamer;
my %Hash;
Dump(\%Hash);
$Hash{'Empty'}[0] = 'no longer';
Dump(\%Hash);
__END__
$HASH1 = {};
$HASH1 = { Empty => [ 'no longer' ] };
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