Thanks -- I did know about the protocol method, but couldn't understand how I could use it before the message went out. Your code does output the correct thing from the debugger, but now I'm getting an error from HTTP::Message that it's having trouble cloning the headers (line 27).

The investigation continues. Thanks!

Update: OK -- going through HTTP::Request means you have to reorganize the parameters from a hashref to a refarray to a flattened hash:

$refhash = { UserId => 533, Something => 'foobart' }
$refarray = [ map { $_, $refhash->{$_} } keys %$refhash ];
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so that the code in HTTP::Headers

sub header
{
    my $self = shift;
    Carp::croak('Usage: $h->header($field, ...)') unless @_;
    my(@old);
    my %seen;
    while (@_) {
    my $field = shift;
        my $op = @_ ? ($seen{lc($field)}++ ? 'PUSH' : 'SET') : 'GET';
    @old = $self->_header($field, shift, $op);
    }
    return @old if wantarray;
    return $old[0] if @old <= 1;
    join(", ", @old);
}
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accepts the data. Now I'm back to getting a 500 Internal Server error from the server I'm hitting .. I'll check back tomorrow when I can talk to the SysAdmin in Europe.