sub read_bytes {
   my ($fh, $to_read) = @_;
   $buf = '';
   while ($to_read) {
      <b>my $bytes_read = read($FH_decrypted, $buf, length($buf));</b>
      die("$!\n") if !defined($bytes_read);
      die("Unexpected end of file\n") if !$bytes_read;
      $to_read -= $bytes_read;
   }
   return $buf;
}
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Should $FH_decrypted be $fh instead? Otherwise the value in $fh being passed to the function is seemingly never used.
Why is the value of $buf set to an empty string? Won't length($buf) be zero and therefore nothing would ever be read?