$ perl -MO=Deparse -p -w -e 's/(\w+)/\u\L$1/'
Useless use of a constant (s/(\w+)/\u\L$1/) in void context at -e line 1.
BEGIN { $^W = 1; }
LINE: while (defined($_ = <ARGV>)) {
    '???';
}
continue {
    die "-p destination: $!\n" unless print $_;
}
-e syntax OK

$ perl -MData::Dump -e " dd\@ARGV " 's/(\w+)/\u\L$1/'
["'s/(\\w+)/\\u\\L\$1/'"]