What if you did not know the names of the keys

Then use the keys builtin to find them out. Something like this:

for my $file (sort keys %messagebin) {
    print "File $file:\n";
    # $messagebin{$file} is a hashref, so:
    my @subkeys = sort keys %{$messagebin{$file}};
        foreach my $subkey (@subkeys) {
           print "\t$subkey = $messagebin{$file}{$subkey}\n";
        }
}
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