lists are passed as values in TCL

list.tcl

#!/usr/bin/env tclsh
proc f {L} {
	puts "f: before: L = $L"
	lappend L 20
	puts "f: after: L = $L"
}

proc g {_L} {
	upvar ${_L} L
	puts "g: before: L = $L"
	lappend L 30
	puts "g: after: L = $L"
}

set L {1 2 3}
puts "initial: L = $L"
lappend L 4
puts "after lappend: L = $L"
f $L
puts "after calling f: L = $L"
g L
puts "after calling g: L = $L"

output

$ ./list.tcl 
initial: L = 1 2 3
after lappend: L = 1 2 3 4
f: before: L = 1 2 3 4
f: after: L = 1 2 3 4 20
after calling f: L = 1 2 3 4
g: before: L = 1 2 3 4
g: after: L = 1 2 3 4 30