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

sub double_it {
   local ($_) = @_;
...
          map { $chars1[$_] . $chars2[$_] }
          0..$#chars1;
}

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

   return join(' ', @pairs);
}

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