If you are willing to preformat your log lines (perhaps with a logging subroutine) you can define two loggers that log to the same file. One has the normal pattern layout, the other has blank space at the beginning of the conversion:
# Untested but I have used similar code
log4perl.appender.file1 = Log::Log4perl::Appender::File
log4perl.appender.file1.filename = 'File.log'
log4perl.appender.file1.mode = append
log4perl.appender.file1.layout = Log::Log4perl::Layout::Patter
+nLayout
log4perl.appender.file1.layout.ConversionPattern = %d{E MMM dd yyy
+y HH:mm:ss} - %m
log4perl.appender.file1 = Log::Log4perl::Appender::File
log4perl.appender.file1.filename = 'File.log'
log4perl.appender.file1.mode = append
log4perl.appender.file1.layout = Log::Log4perl::Layout::Patter
+nLayout
log4perl.appender.file2.layout.ConversionPattern = -
+ %m