Read readdir, -X, sort, map.

$ perl -wl
use strict;
my $dir = q{.};
opendir(DH, $dir) || die "can't opendir $dir: $!";
my %filesize = map { -s "$dir/$_" => $_ } readdir(DH);
closedir DH;
print "$filesize{$_} : $_" for(sort {$b <=> $a} keys %filesize);
__END__
[download]