(tho' the logic of Boolean diadics not returning Boolean values escapes me...)

It allows idioms such as:

$hostname ||= 'localhost';
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Since that is just short for:

$hostname= $hostname || 'localhost';
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it'd set $hostname to just 'true' if Boolean operators returned Boolean values. Of course, the above is short-hand for:

$hostname= 'localhost'  if ! $hostname;
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but the first form is the only one that manages to not repeat $hostname, which can be quite an advantage in some cases, or across a bunch of similar cases. And there are other similar idioms that are useful because of this property.

In any case, it is quite "Perlish".