I finally got around to have a look at this code and the first thing that jumped out and screamed at me was that you are blocking in client_start whilst reading and parsing the CSV input file. What is happening is that the while loop blocks the kernel, whilst you are enqueuing the requests to PoCo-Client-HTTP. These requests are sitting enqueued until the while loop ends and client_start finishes and then in one big whoosh are executed.

As you appear to be using strace to diagnose the problem you are using Linux I guess, so you can use POE::Wheel::ReadWrite and POE::Filters to parse your input file and be a lot more co-operative

Only amendments shown:

use POE qw(Wheel::ReadWrite Filter::Stackable Filter::Line Filter::CSV
+);

# add extra handlers for "file_input" "file_error"

POE::Session->create(
    inline_states => {
        _start                => \&client_start
        , _stop               => \&client_stop
        , got_response        => \&client_got_response
        , transfer_complete   => \&finalize_transfer
        , file_input          => \&file_input
        , file_error          => \&file_error
    }
);

sub client_start {
    my ($kernel, $heap) = @_[KERNEL, HEAP];

    ## $poe_kernel->sig(INT => "_stop");
    
    my $fh = IO::File->new( 'dealermade_pictures.csv', 'r' );

    $heap->{file} = POE::Wheel::ReadWrite->new(
         Handle       => $fh,
         Filter       => POE::Filter::Stackable->new(
                Filters => [ POE::Filter::Line->new(),
                             POE::Filter::CSV->new(),
                ],
         ),
         InputEvent   => 'file_input',
         ErrorEvent   => 'file_error',
    );
   $heap->{_header} = 0;
   return;
}

sub file_input {
   my ($kernel,$heap,$fields) = @_[KERNEL,HEAP,ARG0];

   unless ( $heap->{_header} ) {
       $heap->{_header}++;
       return;
   }

   my ( $picid, $url, $lot, $is_primary ) = @$fields;

   my $temp = generate_tempname( $url );

   if ( -e $temp && -f $temp && -s $temp ) {
       $kernel->post( dmua => request => got_response => HEAD( $url ) 
+);
   }
   else {
       $kernel->post( dmua => request => got_response => GET( $url, Ac
+cept => 'image/*' ) );
   }

   return;
}

sub file_error {
   my ($kernel,$heap) = @_[KERNEL,HEAP];
   delete $heap->{file};
   return;
}
[download]

The reading and parsing of the input file is now asynchronous and HTTP requests are enqueued and happening asynchronously whilst the input file is being read.