use strict;
use Data::Dumper;
use Regexp::Common;

my $name   = qr/[a-z0-9_]+/i;
my $val    = qr/\S+/;
my $braces = qr/$RE{balanced}{-parens=>'{}'}/;
my $node   = qr/($name) \s+ ($braces)/x;
my $assign = qr/($name) [=:] \s* ($val) (?:\s+ | })/x;

my $graph = <<TXT;
graph {
  node { title: "node1" loc {x: 10 y: 20} }
  node {
    title: "node2"
    loc {
      x: 10 y: 20
    }
  }

 edge
 {
   sourcename="node1"
   targetname="node2"
  }
}
TXT

print Dumper( parse_graph($graph) );

sub parse_graph {
    my $graph = shift;

    my $tree = {};
    pos($graph) = 0;
    PARSE: {
        if($graph =~ /\G {? \s* $assign/sgcx) {
            $tree->{$1} = $2;
            redo PARSE;
        } elsif($graph =~ /\G .*? $node/sgcx) {
            push @{ $tree->{$1} }, parse_graph($2);
            redo PARSE;
        }
    }
    
    return $tree;
}

__output__

$VAR1 = {
          'graph' => [
                       {
                         'edge' => [
                                     {
                                       'sourcename' => '"node1"',
                                       'targetname' => '"node2"'
                                     }
                                   ],
                         'node' => [
                                     {
                                       'title' => '"node1"',
                                       'loc' => [
                                                  {
                                                    'x' => '10',
                                                    'y' => '20'
                                                  }
                                                ]
                                     },
                                     {
                                       'title' => '"node2"',
                                       'loc' => [
                                                  {
                                                    'x' => '10',
                                                    'y' => '20'
                                                  }
                                                ]
                                     }
                                   ]
                       }
                     ]
        };