A pipe can be attached to the output stream:

$pssh->push('*', 'command', { stderr_to_stdout => 1,
                              stdout_file => ['|-', 'perl', '-ne', 'pr
+int "%HOST%$_\n"'] },
                              $cmd );
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Or the more efficient:

require POSIX;

my %fh;
for my $host (@hosts) {
  my $pid = open my $fh, '|-';
  if (not $pid) {
    while (<>) {
      print "$host) $_";
    }
    POSIX::_exit(0);
  }
  $fh{$host} = $fh;
  $pssh->add_host($host, default_stderr_fh => $fh, default_stdout_fh =
+> $fh);
}

$pssh->push('*', 'command', $cmd );
$pssh->run;

close $_ for values %fh;
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Note that lines comming from stderr and stdout may arrive intermixed. A more correct solution would be to use different filters for stderr and stdout.