I'm not sure what you mean here - you provide a classic example of where or is most certainly not appropriate (as if you use it, $param will never get the value of $default).

That is,

$param = $value or $default;
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Will result in $param always getting the value of $value, never $default. or has lower precedence than = (in fact it has the absolute lowest precedence of all the operators). Maybe it was just me not being clear, though. You should always use 'or' where what you really want to say is:

($param=$value) or die "darn"
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That is, you don't want the value of die to get assigned to $param (yeah, die doesn't return very often. But it forces scalar context, which is why the alternative is a Bad Thing). Using or, your example would have to be:

$param=$value or $param=$default
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So it's not always a matter of style.

Andrew.