Mushroom wise, I think that would be because of eating the mushrooms. In my experience getting hit with magical effects has some small effect of your perception. Like when I went up a PER level from teleporting. Otherwise I can't think of any logical answer unless there's actually part of the game code that raises PER through fighing and/or dodging.

With the carnivorous plants, yes they do get stronger (and change colour) over time. I had a theory going that they got stronger when you got closer to Genetrix, but usually that takes enough time anyway for the purple plants to show up.

Either that or you're dead by then anyway. Unless you've been praying in Attnam the whole time of course

Oh also in relation to time-based effects, your weapon skills go down without practice over time, as well as AGI and DEX. Maybe some other stats like LStr and AStr but i'm not sure. That would leave you pretty screwed for the rest of the game if you've been praying for days on end in Attnam, unless you plan on getting gods/WIS gain + sci-talk + SoCM to gain artificial limbs to avoid DEX and AGI loss of course. (This also generally more than makes up for lowered weapon skill too)

Hunger, maybe?

Silva quake maybe?

Huh... Well ok then.
Just checked the script, and the only hard-coded monsters in that level are the three different crystal golems. Guess I must've just run into experiment ZQ whatsit on that particular level.

The experiment ZQ...something. I think it spawns down there sometimes.

Note also that when attempting to create the page "Saal'thul" it turns "Saal/'thul" into "Saal//////'thul".

Also be warned of an invisible boss monster down there.

Yes, I just noticed that.
So, i'll go about fixing that and leave you with this:

Saal'Thul is a short sword which is made entirely of mithril. It forces the wielder to become invisible, as long as it isn't broken. It can be mirrored, but not cloned. It is not recommended for use because invisibility raises the danger level but this is possibly outweighed by the fact that invisibility should let you easily kill anything without ESP or Infravision without getting hurt.

Also to take into consideration is that the Wiki doesn't like the word "saal'thul", denying the creation of a page to it.

Unless you've been messing around with the script files (or downloaded the game from somewhere else, possibly), no it wouldn't be bad luck with the level design. The levels are all randomly generated every playthrough and certain measures are in place to make sure important areas are (or aren't...) accessible without special requirements.