if some bonehead set it to -1 previously, the increment op wouldn't be doing the wrong thing.

Oops! No. $| 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
[download]

As you can see, $|++ always sets $| to one (no matter what it was) and $|-- always toggles it.

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