From the code in Mail::Mailer I would expect a Mail::Mailer::smtp::pipe object anyway try to define the WRITE method like this:

{ 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]))
 }
}
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Of course if the glob really is Mail::Mailer::smtp define the function there :-)

If this doesn't help (this is just a wild guess!) try to use Devel::TraceSubs to see what functions are called with what parameters.

Jenda