use strict; 
use warnings; 
use 5.012;

use Parse::RecDescent;

$::RD_ERRORS = 1; #Parser dies when it encounters an error
$::RD_WARN   = 1; #Enable warnings - warn on unused rules &c.
$::RD_HINT   = 1; #Give out hints to help fix problems.
#$::RD_TRACE  = 1; #Trace parsers' behaviour

my $text = <<'END_OF_TEXT';
{ hello }
{{ hello }}
{{{ hello }}}
END_OF_TEXT

my $grammar = <<'END_OF_GRAMMAR';
    {
        use 5.012;
        use Data::Dumper;
    }

    startrule:  brace_block(s)
                { say Dumper(\@item); }

    brace_block:    <rulevar: ($lbraces, $rbraces)>

    brace_block:    lbrace(1..) 
                    { 
                        $lbraces = join '', @{$item[1]};  
                        $rbraces = '}' x length $lbraces;
                    }
                    'hello'
                    "$rbraces"
                    {
                        $return = "$lbraces $item[3] $rbraces";
                    }

    lbrace: / [{] /xms

END_OF_GRAMMAR

my $parser = Parse::RecDescent->new($grammar) 
    or die "Bad grammar!\n";

defined $parser->startrule($text) 
    or die "Can't match text";

--output:--
$VAR1 = [
          'startrule',
          [
            '{ hello }',
            '{{ hello }}',
            '{{{ hello }}}'
          ]
        ];