unicode_strings only ensures that Perl uses character semantics instead of byte semantics for all string operations, which is helpful in the face of ambiguity. (See The "Unicode Bug" in perlunicode.) It doesn't alter the behavior of the length function, which measures the length of a Unicode string in code points, not in grapheme clusters (that is, in real characters).

There's no built-in function in Perl to measure the length of a Unicode string in grapheme clusters rather than in code points.

Read chromatic's article titled New Features of Perl 5.14: unicode_strings for a helpful overview of unicode_strings.