(hope I understand your question correctly)

If a regexp like m/^(.+:)\s*$/ matches a category headline, and you have a logfile like:

Project status:
project created

DB update:
database initialized

Project status:
world domintation ahead

DB update:
database low on memory

DB update:
database borks ; all data lost

Project status:
shattered dreams

Project status:
starting all over again
[download]

then the following program will give you a starting point:

#!/usr/bin/perl

my %cats;
my $cat = "_UNDEFINED:";

open my $fh, '<', 'categories.txt' or die;

while (<$fh>) {
  next if m/^\s*$/;  #skip blank lines
  if (m/^(.+:)\s*$/) {
    $cat = $1;
    next;
  }
  $cats{$cat} .= ">> $_";
}

for (sort keys %cats) {
  print "$_\n";
  print $cats{$_};
}

close $fh;
[download]

The output is:

DB update:
>> database initialized
>> database low on memory
>> database borks ; all data lost
Project status:
>> project created
>> world domintation ahead
>> shattered dreams
>> starting all over again
[download]