$ perl -le'
use warnings;
use strict;

my %stuff = ( "a", 1, "b", 2, "c", 3, "d", 3 );

my %by_size;
for my $key ( keys %stuff ) {
    push @{ $by_size{ $stuff{ $key } } }, $key;
    }

for my $size ( keys %by_size ) {
    print "Size = $size, Names = @{ $by_size{ $size } }";
    }
'
Size = 1, Names = a
Size = 3, Names = c d
Size = 2, Names = b
[download]

There's probably a way to do this while going through the data only once, but I can't think of it. Here's a method that goes through the values (file sizes) once and then through the keys. It saves the number of times each file size is seen, and then prints out the files that have sizes that were seen more than once.

#!/usr/bin/perl

my %sizes = qw( a 1 b 2 c 3 d 3 e 2 f 2);
my %times_seen;

for my $v (values %sizes){
    $times_seen{$v} ? $times_seen{$v}++ : ($times_seen{$v} = 1);
}
for my $k (keys %sizes){
    if($times_seen{$sizes{$k}} > 1 ){
        print "$k => $times_seen{$sizes{$k}}\n";
    }
}
[download]

Oops, I had a mistake there; that last print statement should print $sizes{$k} for the file size, instead of printing the number of times the size was found.

And here's a version that does it while only going through the list once. It does use an extra hash to keep track of matches, though, and it's definitely more complicated, so it probably isn't worth the trouble unless you have a large amount of data -- perhaps enough that reading it all into memory in one hash would be prohibitive, so going through it twice would hit the disk twice.

It works by saving the file sizes to a hash like before, but then when the same size is encountered again, the first file that had that size is printed, and the second one is 'promoted' to a second hash, which is also checked for matches against future sizes. That's the best way I could come up with to print matches as I went, but get the leftover matches printed at the end, and not reprint any.

#!/usr/bin/env perl

my %files = qw( a 111 b 222 c 333 d 333 e 222 f 222);
my %sizes_seen;    # file sizes that have already been seen
my %sizes_matched; # sizes that have already been matched,
                   #   and the last filename with that size

for my $file (keys %files){
    my $size = $files{$file};  # readability variable
    if( $sizes_matched{$size} ){
        print "$sizes_matched{$size} => $size\n";
        $sizes_matched{$size} = $file;
    } elsif( $sizes_seen{$size} ){
        print "$sizes_seen{$size} => $size\n";
        $sizes_matched{$size} = $file;
    }
    $sizes_seen{$size} = $file;
}

# finish printing the remaining matches
for my $s (keys %sizes_matched){
    print "$sizes_matched{$s} => $s\n";
}
[download]