(By the way: You could make the key the filename, depending on what else you were doing in the code. Just use 'values' instead of 'keys' above.)

Not really. You can get the value from the key, but not the other way around. To sort by value you need something like this:

foreach my $hash ( sort {$file_hashes{$a} cmp $file_hashes{$b}} 
        keys %file_hashes ) {
    print "$hash $file_hashes{$hash}\n";
}
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