There's this, or the more fully featured IO::Tee module on CPAN.

package TeeHandle; 

use Carp; 
use Symbol (); 

sub TIEHANDLE { 
  my( $class, $file, $dst_handle ) = @_; 
  my $handle = Symbol::gensym; # Make a filehandle name  
  my $true_handle = Symbol::gensym; 
 
  open( $true_handle, ">&$dst_handle" ) 
    or croak "Can't dup handle '$dst_handle': $!"; 
  open( $handle, ">$file" ) 
    or croak "Can't create '$file': $!"; 
  return bless { handle => $handle, true => $true_handle }, $class; 
} 
 
sub PRINT { 
  my $self = shift; 
  for (@_) { 
    print {$self->{handle}} "$_";  
    print {$self->{true}} "$_";    
  } 
} 

1;

__END__  
package main; 

use Carp; 
use Symbol (); 

tie *STDOUT => 'TeeHandle', "out", 'STDOUT';
tie *STDERR => 'TeeHandle', "err", 'STDERR';
 
print "Testing . . .\n"; 
print STDERR "This goes to STDERR\n";

CORE::warn "Where do I go?\n";

## This doesn't work.
system '/bin/sh', '-c', 'echo "Testing if it\'s propigated" 1>&2';
 
exit 0;
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