use POSIX;

...

my $pid = open(STDOUT, "|-") // die "unable to fork new process";
unless ($pid) {
  while (<>) { print if /the-regular-expression-goes-here/ }
  POSIX::_exit(0);
}

# after this point, STDOUT is filtered
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Update: That works in Unix/Linux systems. I am not sure about Windows, but probably not!