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

use XML::SAX::Writer;

my $DEBUG=0; # set to 1 to debug

my $w = XML::SAX::Writer->new; # see the man for XML::SAX::Writer to output to  file
$w->start_document();

# in your example you don't have a root element, so you need one
my $root= 'root';
$w->start_element( { Name => $root, });

while( <DATA>)
  { # the split will return (char, tag, char, tag, char...)
    # the list always starts and ends with a char
    my @tokens= split( m{\[\[             # [[
                             ([^\]]*)     #   tag
                                     \]\] #      ]]
                         }x
                     ); 

    print join( ' - ', map { "'$_'" } @tokens), "\n" if( $DEBUG);

    while( @tokens)
      { my $chars= shift @tokens;
        $w->characters( { Data => $chars} );

        my $tag= shift @tokens or last;
        if( $tag=~ m{^/(.*)$})
          { $w->end_element( { Name => $1 }); }
         else
          { $w->start_element( { Name => $tag }); }
      }
  } 

$w->end_element( { Name => $root, });
$w->end_document;

__DATA__
[[a]] some text here[[/a]]
[[b]] some more text [[/b]]
[[a]] some text again[[c]]more here..[[/c]]
[[/a]]
[[a]] some text again[[c]]more here..[[/c]][[/a]]