Binary "<=>" returns -1, 0, or 1 depending on whether the left argument
is numerically less than, equal to, or greater than the right argument.
If your platform supports NaNs (not-a-numbers) as numeric values, using
them with "<=>" returns undef. NaN is not "<", "==", ">", "<=" or ">="
anything (even NaN), so those 5 return false. NaN != NaN returns true,
as does NaN != anything else. If your platform doesn't support NaNs then
NaN is just a string with numeric value 0.

    perl -le '$a = NaN; print "No NaN support here" if $a == $a'
    perl -le '$a = NaN; print "NaN support here" if $a != $a'