#!/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";
}