If you're just looking for files having the same size, this example may be sufficient.

...
my %image;
opendir(DIR, $dir) or die("Couldn't open dir $dir: $!");
foreach my $file (readdir(DIR)) {
    my $size = -s "$dir/file_found"; # Simpler size code
    if (exists($image{$size})) {
        handle_duplicate($image{$size}, $file);
    }
    else {
        $image{$size} = $file;
    }
}
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If you want to do something more sophisticated, you may find it easier to build a list of potential duplicates and then postprocess each list in a second loop. The following code will build a hash of arrays, each array containing the filenames that had the same size. It then calls a checking function on each list containing more than one file name:

...
foreach my $file (readdir(DIR)) {
    my $size = -s "$dir/$file";
    push @{$image{$size}}, $file;
}
closedir(DIR);
foreach my $list (values %image) {
    handle_duplicates(@{$list}) if (@{$list} > 1);
}
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