Arduino motor control using digital pins
You can implement PWM using digital pins by turning the pin on and off with respect to your duty cycle, because PWM (Pulse Width Modulation) depends upon the duty cycle the % amount during which the pin is HIGH or LOW

Let say pin 13 of arduino is connected to your l293D enable pin

Sample Code:

void setup()
{
  pinMode(13, OUTPUT);
}

void loop()
{
  digitalWrite(13, HIGH);
  delayMicroseconds(100); // Approximately 10% duty cycle @ 1KHz
  digitalWrite(13, LOW);
  delayMicroseconds(1000 - 100);
}

Details are also on Arduino Help

https://www.arduino.cc/en/Tutorial/SecretsOfArduinoPWM