gistfile1.pl

#!/usr/bin/env perl
use strict;
use warnings;

use Data::Dumper;
use Regexp::Grammars;

my $grammar = qr{
  <nocontext:>
  <Line>
  <rule: Line>		<[Element]>*
  <rule: Element>	<Words> | <Block> | <Command> | <Comment>
  <rule: Command>	<[Words]> ;
  <rule: Block>		\{ <[Element]>* \}
  <rule: Comment>	// .*? \s{2,}
  <rule: Words> 	(?:\b\w+\b) ** \s
}x;

my $string = 'apple{{mango kiwi } guava ; banana; // pear      berry;}';

if ($string =~ $grammar) {
  open my $log, ">", "log.txt";
  print $log Dumper \%/; #/

  print elements($/{Line}{Element});
  
} else {
  die "Did not match";
}

sub elements {
  my @elements = @{ shift() };
  my $indent = shift || 0;
  my $output;

  foreach my $element (@elements) {
    $output .= "\t" x $indent;

    foreach my $key (keys %$element) {
      if ($key eq 'Words') {
        $output .= $element->{$key} . "\n";
      } elsif ($key eq 'Block') {
        $output .= "{\n" . elements($element->{$key}->{Element}, $indent + 1) . ("\t" x $indent) . "}\n";
      } elsif ($key eq 'Comment') {
        $output .= $element->{$key} . "\n";
      } elsif ($key eq 'Command') {
        $output .= join(" ", @{ $element->{$key}->{Words} }) . ";\n";
      } elsif ($key eq 'Element') {
        $output .= elements($element->{$key}, $indent + 1);
      }
    }
  }

  return $output;
}