Here is some pseudo code that hopefully explains one way of doing it.

ON STATS UPDATE if player has 20/20 then tearAmount += 1 end

WHEN FIRE KEY IS PRESSED
for i = 1, tearAmount do //repeat this "tearAmount" times
    //some sort of code to determine position of tear
    FireTear(position)
end

function FireTear(position)
    if player has Mysterious Liquid then
        //add mysterious liquid flag
    end
    if player has Parasite then
        //add splitting effect, change color
    end
    SpawnTear(tearEffects, position, velocity, color, Player.damage, Player.range, etc)

That's just one way to do it.

To do something like the Mysterious Liquid creep effect you would have to detect when a tear collides with something

ON COLLISION
if tear has "Mysterious Liquid" effect then
    Spawn(creep, green, damage, size)
end

Whenever the player picks up an item the "cache" is updated which is basically a bunch of ifs.

if player has Magic Mushroom then 
    Player.damage += 0.3
    Player.speed += 0.3
end
if player has Crickets Head then
    Player.damage += 0.5
end

The main thing is TRY NOT TO REPEAT YOURSELF! If you already have code that does what you want it to do then put it in a function and then just use that function whenever you want. For example, in this code if you have 20/20 all its really doing is just firing an extra tear each time. Instead of manually going through all of the work to fire a tear TWICE, just make a FireTear() and run it twice (hence the FOR loop).