Store your messages as keys in a hash, and increment the count. 
use strict;
use warnings;

my %msgs;
while (<DATA>) {
    s/^\s+//;
    chomp;
    $msgs{$_}++;
}

# Sort by number of occurrences and only show top 8:
my $i = 0;
for my $m (sort {$msgs{$b} <=> $msgs{$a}} keys %msgs) {
    print "$msgs{$m} $m\n";
    $i++;
    last if $i == 8;
}

__DATA__
     Mar  9 08:15:05  gen-vcs11 kernel: kjslah: Unknown symbol unlock_
+page
     Mar  9 08:15:05  gen-vcs11 kernel: kjslah: Unknown symbol generic
+_file_read
     Mar  9 08:15:05  gen-vcs11 kernel: kjslah: Unknown symbol generic
+_file_write
     Mar  9 08:15:05  gen-vcs11 kernel: kjslah: Unknown symbol generic
+_file_mmap
     Mar  9 08:15:05  gen-vcs11 kernel: kjslah: Unknown symbol generic
+_file_sendfile
     Mar  9 08:15:05  gen-vcs11 kernel: kjslah: disagrees about versio
+n of symbol zone_table
     Mar  9 08:15:05  gen-vcs11 kernel: kjslah: Unknown symbol zone_ta
+ble
     Mar  9 08:15:05  gen-vcs11 kernel: kjslahdisagrees about version 
+of symbol unlock_page
Mar  9 08:15:05  gen-vcs11 kernel: kjslah: Unknown symbol unlock_page
     Mar  9 08:15:05  gen-vcs11 kernel: kjslah: Unknown symbol filemap
+_fdatawrite
     Mar  9 08:15:05  gen-vcs11 kernel: kjslah: Unknown symbol find_or
+_create_page

[download]

See also:

perlintro

perldoc -q sort

In reply to Re: How do I read a log file that contents recurring log messages those are separated by newline characters? by toolic
in thread How do I read a log file that contents recurring log messages those are separated by newline characters? by WantToBeJediInPerl

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