I think the main problem was that the subroutine in ::FromCurl is named ->_build_body when it should have been named ->_build_quoted_body. That in turn would make the programming error on my part obvious because the HTTP::Request object was built as:

sub as_request( $self ) {
    HTTP::Request->new(
        $self->method => $self->uri,
        [ %{ $self->headers } ],
        $self->_build_quoted_body(), # was $self->_build_body
    )
};
[download]

... which is obviously wrong. ->_build_quoted_body should only be used when creating Perl code as output.

The fix, on its way to CPAN in version 0.11, is to use the plain ->body:

sub as_request( $self ) {
    HTTP::Request->new(
        $self->method => $self->uri,
        [ %{ $self->headers } ],
        $self->body(),
    )
};
[download]