Part time work is possible. I initially dropped to 4 days/week in lieu of a payrise (comp remained the same). Did that for a while and since then I've been working 2-4 days/week (3 right now) It helped that it was a small company and I had been working there for 5+ years, not sure how I'd go about finding a part time software job otherwise. Everything is negotiable though.
In the US there are likely medical benefit and retirement account requirements for working X hours per week, I think its 32 typically but check with HR or if your place has an employee manual it should be clearly spelled out. To phrase that another way - you may lose medical coverage and/or 401k match depending on the "hours" you work. Scary quotes is because you are likely still salary and not hourly.