I believe you are mistaken. I tried with 5.6.0, 5.6.1, 5.8.0, 5.8.8, 5.8.9 and 5.10.0, but I don't get what you get.

>perl -we"use strict; 1 if func"
Bareword "func" not allowed while "strict subs" in use at -e line 1.
Execution of -e aborted due to compilation errors.

>perl -we"use strict; 1 unless func"
Bareword "func" not allowed while "strict subs" in use at -e line 1.
Bareword "func" not allowed while "strict subs" in use at -e line 1.
Execution of -e aborted due to compilation errors.
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In both case, the fix is to predeclare the sub (sub func;) or use parens in the call (func()). You can't omit parens on a sub call to sub that hasn't been declared.