use strict;
use warnings;
use CGI;

my $hook_called;
my $cgi = CGI->new( \&_hook );

print $cgi->header(),
      $cgi->start_html("FOO"),
      $cgi->start_multipart_form,
      $cgi->filefield('file'),
      $cgi->submit,
      $cgi->end_form;

sub _hook
{
    my ( $filename, $buffer, $bytes_read, $data ) = @_;
    if( !$hook_called )
    {
        print "Content-Type: text/html; charset=ISO-8859-1\n\n";    # $cgi is not defined here
        $hook_called = 1;
    }
    else
    {
        print "Read $bytes_read bytes of filename\n";
    }
    $hook_called += $bytes_read;
}

print $cgi->end_html;