I'm all for simplicity, so I'd split this into two steps:

my $data = '<%def .errors>
missing_name: You must provide your name.
missing_email: You must provide your email address.
</%def>';

my ( $errors ) =
  $data =~ m[\Q<%def .errors>\E\n
             (.*\n)
             \Q</%def>\E
           ]sx;
my %errors = $errors =~ m{(\w+):\s*(.*?)\n}g;

print Dumper \%errors;
[download]

Output:

$VAR1 = {
          'missing_email' => 'You must provide your email address.',
          'missing_name' => 'You must provide your name.'
        };
[download]