use strict;
use warnings;

use Data::Dumper;

my %hash;
my $header;

while (my $line = <DATA>) {
    chomp $line;

    my ($k, $v) = split ':', $line;

    next if ! $k;

    if (! $v) {
        $header = $k;
        next;
    }

    $hash{$header}{$k} = $v;
}

print Dumper \%hash;

__DATA__
first:
this:that
here:there
when:what
how:where
now:later

second:
this:that
here:there
when:what
how:where
now:later