The uppercase aliases for the string comparison operators were finally removed in release 5.7.0 - see perldelta. I'm not sure when they were deprecated however. Why this gives the error you see I don't know.

Interesting... in 5.6.1 (ActiveState build 635) I get the "not enough arguments" (open without brackets) or plain old "syntax error" (open with brackets), but I don't get the deprecation warning.

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I'd like to be able to assign to an luser