my $delta = 0.01;  # choose some sensible value (defines range)

    for my $i (0 .. $#array_of_x_values) {
      
        my ($x, $y) = ($array_of_x_values[$i], $array_of_y_values[$i]);

        my $y_trend = $intercept + ($x * $slope);
        my $resid   = $y - $y_trend;

        if ($resid > -$delta and $resid < $delta) {
            # points are "on" (i.e. within some small range around) trend line
            # ...
        } else {
            if ($resid > 0) {
                # points are above the trend line
                # ...
            } else {
                # points are below the trend line
                # ...
            }
        }
    }