I tried paragraph mode, still didn't need /s and /m. :)

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11123126
use warnings;

local $/ = ''; # paragraph at a time
my %hash = map { $1 x /(\w+):\n/, { /(\w+):(\w+)/g } } <DATA>;
use Data::Dump 'dd'; dd \%hash;

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

second:
this:that
here:there
when:what
how:where
now:later
name:onlysecond
[download]