See the current Perl documentation for uc.
Here is our local, out-dated (pre-5.6) version:
uc - return upper-case version of a string
Returns an uppercased version of
EXPR. This is the internal function implementing the
\U escape in double-quoted strings. Respects current
LC_CTYPE locale if
use locale in force. See the perllocale manpage.
EXPR is omitted, uses