I'm not as clever as some, so I did it the long way...

#!/usr/bin/perl

use strict;
use warnings;

while( my $ln = <DATA> )
  {
    chomp( $ln );

    my @flds = split( '@', $ln );
    foreach ( @flds )
      {
        if ( /(#*)/ )
          { 
            print "/$ln/ --> /$1/\n" if ( length( $1 ) % 2 ) == 1;
          }
      }
  }

__DATA__
#@##@###@####@#####@
#@##
###@####
#####@
[download]

This results in:

/#@##@###@####@#####@/ --> /#/
/#@##@###@####@#####@/ --> /###/
/#@##@###@####@#####@/ --> /#####/
/#@##/ --> /#/
/###@####/ --> /###/
/#####@/ --> /#####/
[download]

Revised: This won't work for a string with no pound signs. You'd need to modify the length test to include  || length( $1 ) == 0.