It's not so much a buffering issue as you waiting for the child to finish before printing anything. Just use system instead of backticks.

my $rsync_cmd = "rsync -vza --delete --delete-excluded $exclude_opts $
+conf{dir_backup_from} $mountpoint_reversed_enc";

system($rsync_cmd);
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rsync is still free to buffer its output and there's nothing you can do about that, but since its STDOUT is connected to a terminal (assuming the Perl script's STDOUT is connected to a terminal), it'll probably avoid buffering its output.

You have another unrelated problem if $conf{dir_backup_from} contains the path to a directory rather than bourne shell argument literal, one that can easily be fixed by using the multi-argument form of system.

my @rsync_cmd = (
   'rsync',
   '-vza',
   '--delete',
   '--delete-excluded',
   split(' ', $exclude_opts),
   $conf{dir_backup_from},
   $mountpoint_reversed_enc,
);

system(@rsync_cmd);
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