sub MY::postamble {
  my (undef,%h) = @_;
  my $ret = "bench :: $h{BENCHMARKS}\n";
  $ret .= <<'POSTAMBLE';
  $(NOECHO) /usr/bin/env perl '$<'
POSTAMBLE
  return $ret;
}
WriteMakefile(
  INSTALL_BASE      => blablabla
  ...
  postamble => { BENCHMARKS => 'benchmarks/*.b' }
);