You really need a parser. Using extended regexp features, you can write a parser in a regexp.

use strict;
use warnings;

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

# Perl code in regexps close over lexicals when
# the regexp is compiled. It's best to avoid
# using lexical variables declared outside the
# regexp in Perl code in regexps.
local our %errors;
$text =~ /
   (?{ +{} })
   <%def\ \.errors> \n
   (?:
      (\w+): \s* (.*) \n
      (?{ +{ %{$^R}, $1 => $2 } })
   )*
   <\/%def> \n
   (?{ %errors = %{$^R} })
/x
   or die("Bad text\n");

for (keys %errors) {
   print("$_ => $errors{$_}\n");
}
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5.10 has features that make this even easier, but I haven't taken the time to look at them yet.