#!/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;