I don't think AttrIndent does what you expect.

XML::Simple:

AttrIndent => 1 # out - handy

When you are using XMLout(), enable this option to have attributes printed one-per-line with sensible indentation rather than all on one line.

Update 1: Args to XMLout is a list, not a hash, as pointed out by Anonymous Monk.

Update 2: Using use XML::Simple qw(:strict); would have given a warning:

No value specified for 'ForceArray' option in call to XMLin()
[download]

use strict;
use warnings;
use XML::Simple;

my $str = <<EOL;
<opt class="java.beans.XMLDecoder" version="1.6.0-oem">
  <object class="java.util.HashSet">
    <void method="add" string="baseball" />
    <void method="add" string="football" />
  </object>
</opt>
EOL

my $ref = XMLin($str);
open FH, ">", "webcams.xml" or die ("open failed");
print FH XMLout($ref, 
  AttrIndent => 1,
  xmldecl => '<?xml version="1.0" encoding="UTF-8"?>');
close FH or die ("close failed");
__END__
<?xml version="1.0" encoding="UTF-8"?>
<opt class="java.beans.XMLDecoder"
     version="1.6.0-oem">
  <object class="java.util.HashSet">
    <void method="add"
          string="baseball" />
    <void method="add"
          string="football" />
  </object>
</opt>
[download]