my @ordered;
while ( <DATA> ) {
  $ordered[$1] = $2 if /input datafile file number=(\d+) name=(\S+)/
}
print "$_\t$ordered[$_]\n" for 1 .. $#ordered;

__DATA__
Starting backup at 2011-05-31 02:00:05
channel ch1: starting compressed full datafile backup set
channel ch1: specifying datafile(s) in backup set
input datafile file number=00010 name=/foo/oradata/bar/foodb-lob01.dbf
input datafile file number=00004 name=/foo/oradata/bar/foodb-data02.dbf
input datafile file number=00007 name=/foo/oradata/bar/undotbs02.dbf
channel ch1: starting piece 1 at 2011-05-31 02:00:06
channel ch2: starting compressed full datafile backup set
channel ch2: specifying datafile(s) in backup set
input datafile file number=00003 name=/foo/oradata/bar/tools01.dbf
input datafile file number=00006 name=/foo/oradata/bar/foodb-index11.dbf
input datafile file number=00002 name=/foo/oradata/bar/undotbs01.dbf
channel ch2: starting piece 1 at 2011-05-31 02:00:06
channel ch3: starting compressed full datafile backup set
channel ch3: specifying datafile(s) in backup set
input datafile file number=00008 name=/foo/oradata/bar/foodb-data01.dbf
input datafile file number=00009 name=/foo/oradata/bar/foodb-index01.dbf
input datafile file number=00005 name=/foo/oradata/bar/xml01.dbf
input datafile file number=00001 name=/foo/oradata/bar/system01.dbf
channel ch3: starting piece 1 at 2011-05-31 02:00:07
channel ch1: finished piece 1 at 2011-05-31 02:34:54
#----------------8<----------------