thank you, really thank you, dude

For that you will need to write a whole vscript, which consist of squirrel coding language with all these functions:
https://developer.valvesoftware.com/wiki/Left_4_Dead_2/Scripting/Script_Functions

Unfortunately i do not know enough about Squirrel to be of any use to you.
But there's a few people in D4M who could help you.

it doesn't work. how to write it so that it activates when you are damaged?

alias lens_eboss "r_screenoverlay dirt_eboss/dirt"
lens_eboss

the one on the bottom cause the alias to run.

could you show an example here in the comments how you would write it?
I wrote it like this:
exec lens_dirt
alias lens_eboss "r_screenoverlay dirt_eboss/dirt"
exec lens_eboss "r_screenoverlay dirt_eboss/dirt"
exec lens_eboss
exec r_screenoverlay dirt_eboss/dirt

none of these examples work

"But the problem would be that it would execute the script upon game load instead of each and every map load."

does source have a command that loads scripts by priority? sequentially
by the way, we are talking about HL2

By writing the "lens_eboss" or any of the other alias names after you defined them, it would automatically run these. But the problem would be that it would execute the script upon game load instead of each and every map load.

There's that "scripts\vscripts\director_base_addon.nut" Vscript that you could use (make one of your own). Those are loaded on every map load and could be used to set up the screen overlays. But I don't know what you'd need to write to make it do that.
Perhaps there's already a screen overlay mod (like gas mask or whatever) that you could use as a base.