{ package Mail::Mailer::smtp::pipe;
 sub WRITE
 {
  my $fh = $_[0];
  print $fh substr($_[1],0,$_[2])
 }
}
# taken from Tie::Handle
or
{ package Mail::Mailer::smtp::pipe;
 sub WRITE
 {
  my $fh = $_[0];
  $fh->PRINT(substr($_[1],0,$_[2]))
 }
}
or maybe 
{ package Mail::Mailer::smtp::pipe;
 sub WRITE
 {
  my $fh = $_[0];
  $fh->format_write(substr($_[1],0,$_[2]))
 }
}