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