mousestate=1 will do the trick. I cant think offhand how you would disable the weapon from firing but pretty sure its do-able.
There are probably a couple of different ways to either put away a weapon or stop from firing if conditions are met.
hideplrweapon for instance.
To make things even simpler you could use right mouse button. mousestate=2.
Without knowing what you want to do I can only guess at the script.
For example to use say a gradual health up:
;Artificial Intelligence Script
;Header
desc = health up left mouse
:state=0,plrdistwithin=80,pickobject=1:hideplrweapon,state=1
:state=1,pickobject=0:state=0
:state=1,mousestate=1,plrhealthless=99:setframe=0,plraddhealth=5,state=2
:state=1,mousestate=1,plrhealthgreater=99:state=10
:state=2:incframe=0,state=3
:state=3,frameatend=0:state=4
:state=4,plrdistfurther=90:state=0
;End of Script
This would go into a health pack entity or one of those medicine terminals or whatever, when the pack is centre screen clicking on it would gradually increase health, of course you would have to reselect your weapon when done.
I just threw it up so not sure it would work without testing but you get the idea.
Awesome! Its one of those threads.