sub double_it {
   return join ' ', $_[0] =~ /\w(?=\w(?<=(..)))/g;
}

##</code><code>##

sub double_it {
   local ($_) = @_;

   (my @chars1) = (my @chars2) = split //;
   pop   @chars1;
   shift @chars2;

   return join ' ',
          grep { !/\s/ }
          map { $chars1[$_] . $chars2[$_] }
          0..$#chars1;
}

##</code><code>##

sub double_it {
   local ($_) = @_;

   my @list = /(\w)(?=(\w))/g;

   my @pairs;
   push(@pairs, shift(@list).shift(@list))
      while @list;

   return join(' ', @pairs);
}

##</code><code>##

sub double_it {
   our @pairs; local *pairs;
   () = $_[0] =~ /(\w)(?=(\w))(?{ push(@pairs, "$1$2"); })/g;
   return join(' ', @pairs);
}