If for some reason $| = -1,...

No. Please don't make assertions like that when you haven't tested them.

$| is magic. It always equals either zero or one. Nothing else. Behold:

print "$|\n";      # 0

$| = -1;
print "$|\n";      # 1

$|++;
print "$|\n";      # 1

$|++;
print "$|\n";      # 1

$|--;
print "$|\n";      # 0

$|--;
print "$|\n";      # 1

$|--;
print "$|\n";      # 0
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As you can see, $|++ always sets $| to one (no matter what it was) and $|-- always toggles it.

I agree that $| = 1; is more clear to newer programmers. But rationalle for that usage has to do with the learning curve and not with the behavior of $|.

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"Perl is a mess and that's good because the
problem space is also a mess." - Larry Wall