ForceContent => 1, I think. Of course, the result of XMLin will be different than what you presented.

Update: Yup.

use strict;
use warnings;

use XML::Simple qw( );

my $xml = <<'__EOI__';
<doc>
    <Last>Smith</Last>
    <First>First</First>
</doc>
__EOI__

my $xs = XML::Simple->new(
   ForceContent => 1,
);

print(
   $xs->XMLout(
      $xs->XMLin($xml)
   )
);
[download]

<opt>
  <First>First</First>
  <Last>Smith</Last>
</opt>
[download]