My initial understanding of the OP's question was that it has to do with Unicode being able to represent the same user-visible character in multiple different ways, like with combining characters. That is, the two strings "\N{LATIN SMALL LETTER E WITH ACUTE}" and "e\N{COMBINING ACUTE ACCENT}" report different lengths (1 resp. 2), even though on the screen they both look like "é" (one "grapheme"), and so users would expect a "length" of each string to be reported as 1. I may have misunderstood the OP's question though - if you have the strings "ffi" vs. "ffi", and you want to know if they have the same length and/or are equal, then perhaps what the OP is looking for is Unicode equivalence (normalization).

use Unicode::Normalize;
use Data::Dump;
dd NFD("\N{LATIN SMALL LETTER E WITH ACUTE}"),
   NFD("e\N{COMBINING ACUTE ACCENT}");
dd NFC("\N{LATIN SMALL LETTER E WITH ACUTE}"),
   NFC("e\N{COMBINING ACUTE ACCENT}");
dd NFKD("\N{LATIN SMALL LIGATURE FFI}");
__END__
("e\x{301}", "e\x{301}")
("\xE9", "\xE9")
"ffi"
[download]

Updated example code to include the "é" examples.