The Widows Wine can be obtained after defeating the Spider Boss. To fight the Spider Boss you must first obtain the KT-4 weapon to destroy the blue webs behind Lab A (near the blue water pool).
To defeat the Boss keep shooting it in its face while it's spitting poison. The poison will damage you if you stand in it and the Boss will also smack its legs down. Every so often it will also spawn the smaller spiders which will attack you. Once you defeat the Boss simply take the path on the right and you will pass by a purple pool of water. Stand in the water and Hold X to drink the Widows Wine and obtain the perk.Note:
Make sure you aren't full on perks as you won't be able to take the Widows Wine. The maximum number of perks is 4 unless you have something allowing you to take a 5th (Gobblegum or challenge reward). Perk-a-holic might aslo void this.