Perl's uc complies with Unicode's uppercasing rules ("when Unicode rules are in effect")

Case translation operators use the Unicode case translation tables.

Ref:
latest perlunicode
latest perlunicode at time of writing

So uc("ß") will change if and when Unicode changes.

The latest version of Unicode (14.0.0) indicates the uppercase of "ß" (LATIN SMALL LETTER SHARP S) is "SS". (The fourth field.)

00DF; 00DF; 0053 0073; 0053 0053; # LATIN SMALL LETTER SHARP S
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Ref:
Latest case special case rules
Latest case special case rules at time of writing

Seeing as the ẞ (LATIN CAPITAL LETTER SHARP S) has been part of Unicode since version 5.1 in 2008, this isn't likely to change.