#!/usr/bin/env perl

use strict;
use warnings;

my %data;

{
    local $/ = ":\n";
    my $block_re = qr{\A(.*)^(.*?):?\Z}ms;
    my $key;

    while (<DATA>) {
        my ($data, $label) = /$block_re/;
        $data{$key} = $data if defined $key;
        $data{$key} .= $label if eof DATA;
        $key = $label; 
    }
} 

use Data::Dump;
dd \%data;

__DATA__
AAA:
123
456
789
BBB:
qwe
rty
uio
CCC:
asd
fgh
jkl