opendir(my $dirh, ".");
my @all = grep/\.csv(\.xlsx?)?$/i, readdir $dirh;
my %excel; 
@excel{map lc, grep /\.xlsx?$/i, @all} = ();
for(grep $_ !~ /.xlsx?$/i, @all) {
    say unless exists $excel{lc "$_.xlsx"} || exists $excel{lc "$_.xls"};
}