You could use the line minus the date as a hash key, incrementing the value each time you encounter the line (with a different date).

my %count;
while (<>) {
  chomp($_);
  $count{substr($_, 9)}++;
}
[download]

Now iterate a second time to divide by the appropriate values in %count and write those to a file.

It is assumed that the lines are identical in format/whitespace and that the dates are always 8 characters wide (if not, use a regexp to extract).

Hope this helps, -gjb-