. . .
  open( my $fh, '>', 'myprogressdata') or die "bad open: $!";
  $fh->autoflush; # no buffering, print writes immediately 
  my $q = CGI->new( \&myhooksub, $fh );
  # You'll get control here only after CGI.pm is done
  # processing form and upload data
  . . .