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
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