#!/usr/bin/perl -w
use strict;

my $test="xyz,[a,b],c,'d',[e,[f,g,h,[i,j],k,l],m,n],o,p";

my @test = $test =~ m/,|[\w']+|[\[\]]/g;
my @result;
while (@test)
{
   my $t = shift @test;
   if ($t =~ /[\w']+/) { push (@result, $t); } #simple "word" thing
   elsif ($t =~ /\[/)              #complex bracketed expression
   { 
      push (@result, bracket_exp());
   }
   # commas are skipped..
}

sub bracket_exp
{
   my $temp ="[";
   my $depth = 1;
   while ($depth)
   {
     my $next_thing = shift @test;
     $temp .= $next_thing;  
     $depth++ if $next_thing eq '[';
     $depth-- if $next_thing eq ']';
   }
   return ($temp);
}

foreach my $t ( @result)
{
   print "$t\n";
}

__END__
prints:
xyz
[a,b]
c
'd'
[e,[f,g,h,[i,j],k,l],m,n]
o
p