Ich hab(e) können is the perfect of können, with Ersatzinfinitiv können instead of the expected past participle gekonnt. Whether Ersatzinfinitiv occurs or not is subject to some variation; when another infinitive is present – runterspülen in this instance – it is expected. The sentence is regular Standard German (although elision of the accusative object of runterspülen is somewhat rare).
Ich habe (es) runterspülen können.
I was able to wash (it) down.
Putting runterspülen in first position makes it the topic.
Note that können + infinitive perfect is completely different: This would be the epistemic use of the modal.
Ich weiß nicht, ob der Kollege noch da ist. Er kann auch schon Feierabend gemacht haben.
I'm not sure whether my colleague is still here. I think it's possible that he went home already.
As opposed to:
Er hat heute früher Feierabend machen können.
He was able to go home early today.