use strict;
use Win32::OLE qw(in);
use Win32::OLE::Const 'Microsoft Office';

my $Doc = Win32::OLE->GetObject('g:\scratch\test.doc');

if ( ( 0 + Win32::OLE::LastError() ) != 0 ) {
  $Doc->CustomDocumentProperties->Add(
    {
      Name          => 'A custom property',
      LinkToContent => 0,
      Type          => msoPropertyTypeString,
      Value         => "Whatever"
    }
  );
}