I kind of like this solution, but YMMV. The approach isn't much different from yours.

my @stack = ({});
while (<DATA>) {
   s/\s+$//; # remove trailing space and newline
   my $depth = 0;
   $depth++ while $depth < @stack && s/^[|` ][- ] //;
   die 'malformed input' if $depth < 1;
   splice @stack, $depth;
   my $obj = $stack[-1];
   if (/^(.*?) = ?(.*)/) {
      $obj->{$1} = $2 eq 'undef' ? undef : $2;
   }
   elsif (s/ \(no elements\)$//) {
      $obj->{$_} = [];
   }
   else {
      push @stack, $obj->{$_} = {};
   }
}
my $output_hash = $stack[0];
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