Perl does not have structs (i.e. data that has a fixed set of named slots). The nearest equivalent would be a locked hash:

use Hash::Util qw( lock_keys );

my %doc = ( filename => '', fileID => undef, tags => [] );
lock_keys(%doc);
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If you look behind the mask of a MooX::Struct, you'll find that it is just a hash and doesn't have the fixed keys property you may expect from real structs e.g.

%> reply
0> use MooX::Struct -rw, Document  => [ qw( $fileID $filename @tags ) 
+];
1> my $doc = Document[ 1, "PetsFile"];
$res[0] = bless( {
         'fileID' => 1,
         'filename' => 'PetsFile'
       }, 'MooX::Struct::__ANON__::0001' )

2> $doc->{foo} = 'bar';
$res[1] = 'bar'

3> delete $doc->{fileID};
$res[2] = 1

4> $doc
$res[3] = bless( {
         'filename' => 'PetsFile',
         'foo' => 'bar'
       }, 'MooX::Struct::__ANON__::0001' )

5>
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I used reply but you could also use Data::Dumper to do the inspection.