my @words = qw/FrobniZZ Shiftboard CocaCola ShiftBoard/;
my %cases_found;
for my $word (@words) {
    push @{ $cases_found{ lc $word } }, $word;
}

for my $lc_word ( sort keys %cases_found ) {
    print "$lc_word found with these cases: ",
        join( ', ', @{ $cases_found{ $lc_word } } ), "\n";
}