#!/usr/bin/perl

use warnings;
use strict;
use Data::Dumper;
my $input = '(a,b,(c,d,(e,f,g)))';

print Dumper parse_expression($input);

sub parse_expression {
    my $exp = shift;
    my @tokens = ();
    $exp=~s/\s*([()])\s*/ $1 /go;
    # Get tokens
    push @tokens, $1 while $exp=~/\G\s*(".*?")/gc or $exp=~/\G\s*('.*?')/gc or $exp=~/\G\s*(\S+)/gc;
    
    # Find any parens. 
    my (@lp,@rp) = ();
    for (my $p =0; $p < @tokens; $p++){
        if ($tokens[$p] eq '('){
            push @lp,$p;
        }elsif($tokens[$p] eq ')'){
            push @rp,$p;
        }
    }
    
    if ( @lp != @rp){
        warn "Mismatched parens in expression.\n";
        return;

    }

    my @temp = @tokens;
    for (my $i=0; $i < @rp; $i++){
         my @wanted;
         for (my $j = $#lp; $j >= 0 ; $j--){
            if ( defined $lp[$j] && $lp[$j] < $rp[$i] ){
                (undef,@wanted) = @tokens[ $lp[$j] ..  ($rp[$i] - 1 ) ] ;    
                @tokens[ $lp[$j] ..  ($rp[$i]) ] = [ grep {defined $_ } @wanted];
                push @temp, map {split /\s*,\s*/} @wanted;
                $lp[$j] = $rp[$i] = undef;
                last;
            }
         }
    }

  return $tokens[0];

}

__DATA__
# OUTPUT
$VAR1 = 
          [
            'a,b,',
            [
              'c,d,',
              [
                'e,f,g'
              ]
            ]
          ];