$ cat t.pl
#!/usr/bin/perl

my %hash = (key1=>10,key2=>10,key3=>3,key4=>5,key5=>10);

my %invhash;
while (my ($k,$v) = each %hash) {
    push @{$invhash{$v}}, $k;
}

for my $v (sort {$a<=>$b} keys %invhash) {
    print "$v cnt=", scalar(@{$invhash{$v}}), 
          " keys=",join(", ",@{$invhash{$v}}), "\n";
}

$ perl t.pl
3 cnt=1 keys=key3
5 cnt=1 keys=key4
10 cnt=3 keys=key5, key2, key1