Parsing with a state variable ($category in this case) is one way to remember which heading th text falls under:

use Data::Dumper;
use strict;
use warnings;

my %parse_tree;

my $category;
while (my $line = <DATA>) {
    if ($line =~ /^(\w+:)$/) { 
        $category = $1;
        $parse_tree{ $category} = [];
    }
    else {
        push @{$parse_tree{ $category}}, $line;
    }
}

print Dumper( \%parse_tree);

__DATA__
heading1:
text1
text2
text3
heading2:
text4
text5
text6
[download]

yields

$VAR1 = {
          'heading1' => [
                          'text1
',
                          'text2
',
                          'text3
'
                        ],
          'heading2' => [
                          'text4
',
                          'text5
',
                          'text6
'
                        ]
        };
[download]

In the dumped hash, note that the newlines are preserved.

Update: altered the regex to capture the colon.