Unexpected TCL error from the return statement

It's a little odd because this appears to actually work with multiple headers in 11.3:

when HTTP_REQUEST {
    if { [string tolower [HTTP::header screen]] equals "abc" } {
        log local0. "match"
    }
}

What version are you on?

In lieu of the above, you could also iterate through the list of headers like this:

when HTTP_REQUEST {
    foreach x [HTTP::header names] {
        if { ( $x equals "screen" ) and ( [string tolower [HTTP::header $x]] equals "abc" ) } {
            log local0. "match"
            break
        }
    }
}