correct syntax for boolean expressions

Try this:

set var_1 [expr { [info exists var_2] || [info exists var_3] }]

By the way, Nitass is probably using the tclsh shell in the BIG-IP management GUI. You can also write pure TCL scripts with this:

!/usr/bin/tclsh

set var_2 true
set var_3 false

puts [info exists var_2]
puts [info exists var_3]

set var_1 [expr { [info exists var_2] || [info exists var_3] }]

puts $var_1

Then chmod the script:

chmod 755 balto.tcl

And run it like any other Bash script:

./balto.tcl

Great for troubleshooting TCL logic outside of an iRule.