Unroll the condition and duplicate the process (or subroutine): 

if ( $y[i] <= $y ) {
    if ( $y < $y[$j] ) {
        # process ...
    }
}
elsif ( $y >= $y[$j] ) {
    # the same process ..
}

[download]
Or a simple calculation: 
if ( ( $y[$i] - $y ) * ( $y - $y[$j] ) > 0 ) {
    # process

[download]
Remember: Memoize is your friend.

In reply to Re: Speeding up point-in-polygon -- take two by explorer
in thread Speeding up point-in-polygon -- take two by punkish

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