What diotalevi said; and in the case of these particular functions, you can look at the desired semantics and do something like:

return 0 unless @_;
return $var1 if @_ == 1;
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This may seem like a lot of work to you, but at least it makes explicit the behavior of your code with, um, nonstandard arguments.

And in regard to ||=, personally I think it's a pretty elegant operator, but perhaps you might like this better:

my $var1 = shift || 0;
my $var2 = shift || 0;
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Again, this makes things *more* explicit than they were. Maybe that isn't what you had in mind when you said "better" :)