How to do a configuration on RMAN with run block without save this configuration on rman I want to do a run block with this configuration for test:

run {
CONFIGURE CHANNEL DEVICE TYPE DISK MAXPIECESIZE 50M;
BACKUP DATABASE;
}


But I don't want that the configuration CONFIGURE CHANNEL DEVICE TYPE DISK MAXPIECESIZE 50M stay save on rman after of run the run block, how to do it?
Instead of CONFIGURE, just use ALLOCATE to allocate a channel with the desired parameters. You will also have to give a name for the channel, which is C1 in the example here:

run {
ALLOCATE CHANNEL C1 DEVICE TYPE DISK MAXPIECESIZE 50M;
BACKUP DATABASE;
}