How to assign global variable under a procedure built in of forms personalization?

Hi All,
I am trying to execute a procedure builtin action of form personalization but facing issues while assigning a value to global variable.

Code is as  below :
 
='begin
IF '''||${global.xx_sys.value}||''' = '''||${global.XX_START.value}||''' then
:global.XX_TEMP := 234;
END IF;
End'

IF clause is working properly, but the assignment is failing with error
"ORA-01008 Not all variables bound"

I have tried '''||:global.XX_TEMP||''' but the actual variable value is retrieved eg. 123 and the assignment is failing.
What is the syntax to assign a global variable in a forms personalization procedure?