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.