Got it....

It was variable interpolation for a username with an '@' in it.

use LWP::UserAgent;

my $target = 'sharegeo';  # opendepot, jorum, sharegeo

my ($host, $username, $password, $realm, $sword_url, $collection) = ()
+; 
for ($target) {
  /opendepot/ && do {
     $host = 'devel.edina.ac.uk:1201';
     $collection = '/sword-app/deposit/archive';
     $username = 'myUsername';
     $password = 'myPassword';
     $realm = 'SWORD';
     last;
  };
  /jorum/ && do {
     $host = 'dev1.jorum.ac.uk';
     $collection = '/sword/deposit/123456789/15';
     $username = 'me%40this.domain';
     $password = 'myPassword';
     $realm = 'SWORD';
     last;
  };
    /sharegeo/ && do {
     $host = 'devel.edina.ac.uk:9248';
     $collection = '/sword/deposit/123456789/2';
     $username = 'me%40that.domain';
     $password = 'anotherPassword';
     $realm = 'SWORD';
     last;
  };

}

# file to send via POST:
my $filename = "Broker_test_package.zip"; # A DSpace/Mets zip file

my $file = "";
open(FILE, "$filename" ) or die('cant open input file');
binmode FILE;
while(<FILE>) { $file .= $_; }
close FILE;

my $ua = LWP::UserAgent->new();

my $req = HTTP::Request->new( POST => 'http://'.$username.":".$passwor
+d."@".$host.$collection );

#my $length = do {use bytes; length $file};

# Tell SWORD to process the contents of the zip file as DSpaceMets
$req->header( 'X-Packaging' => 'http://purl.org/net/sword-types/METSDS
+paceSIP',
              'X-No-Op' => 'false',
              'X-Verbose' => 'true',
              'Content-Disposition' => "filename=$filename"
);
$req->content_type( 'application/zip' );

$req->content( $file );
#$req->content_length($length);

# Et Zzzzooo!
my $res = $ua->request($req);    

if ($res->is_success) 
{
    print $res->content;
}
else 
{
    print $res->status_line, "\n";
}
[download]

.... and that works for both a perl application (EPrints) and a java one (DSpace) Thanks to those who helped (on-line and off-line)