The include and must_include variables specify a file that contains text that should be included in the script. The entire script can thus be put in a file. This is a a user-defined variable.
The difference between include and must_include is that include will fail silently if the file is not found during script reload, while must_include will throw an error if the file is not found.
A gotcha for must_include is that it appears to be invoked on parse not execution so even if you have it in a subroutine or an if...then...else block you always need to have the specified file.
