I have a Plundering Poro on Starspring and a hidden card. I reveal the hidden card and play Evelynn. Now her "When you play me" triggers get added to the chain and Starspring gets added to the chain. Can I first target my Poro with Starspring, then charm an enemy with Evelynn—first in, last out—and kill a unit from my enemy, and then move my Poro?

I couldn't find a card called "Starspring," but based on your description and the "Star Spring" battlefield, I can clarify how the chain and resolution work for your scenario.

The Ruling

You cannot interleave the execution of two separate triggered abilities in the way you described. Because both abilities (Evelynn's "When you play me" and Star Spring's triggered ability) are placed on the chain, you must resolve them one at a time, completely.

1. Chain Construction: When you play a card (Evelynn) and trigger an effect (Star Spring), both are added to the chain as items. You decide the order in which they are added (First-In, Last-Out logic applies to the resolution).
2. Resolution (LIFO): The chain resolves in Last-In, First-Out order. You must fully resolve the last item added to the chain before you move on to the next one.
3. Targeting: You cannot start one ability, interrupt it to use another, and then return to the first. When you resolve an ability, you perform all instructions provided by that ability at once.

Why this doesn't work

If you have both triggers on the chain, you have two options for the order:

• Order A (Star Spring last): Resolve Star Spring completely (move a unit), then resolve Evelynn's trigger completely (move an enemy).
• Order B (Evelynn last): Resolve Evelynn's trigger completely, then resolve Star Spring completely.

You cannot "target my poro with Star Spring, then charm with Evelynn's ability." Your mention of Charm adds a third layer, but Charm is a spell. If you played it as a Reaction to these triggers, it would go on top of the chain and resolve before the triggers, but it still does not allow you to partially resolve the triggers themselves.

Summary: You must resolve each chain item (ability or spell) fully before starting the next. You cannot mix instructions from two different triggers or spells mid-resolution.

This is my interpretation based on Rule 326.1 (LIFO resolution) and Rule 359.3 (Chain resolution). No official FAQ exists for this specific combination of triggers.