use warnings;
use strict;

my $password = "hide_me";
my $log_file = "test.log";
open STDOUT, "| perl | tee -a $log_file";
select STDOUT;
$| = 1;

print <<_END_FILTER_SCRIPT_;
  use warnings;
  use strict;
  select DATA; \$|=1;
  select STDOUT; \$|=1;
  while (<DATA>) {
      s/\Q$password\E/removed/ig;
      print;
  }
  __DATA__
_END_FILTER_SCRIPT_

print "The password is $password\n";
foreach (qw{Output is unbuffered if these words are printed one at a time.}) {
        print $_, " ";
        sleep 1;
}
print "\n";
sleep 1;
print "Output is line buffered if this appears one second after the last line.\n";
close STDOUT;

##</code><code>##

The password is removed
Output is unbuffered if these words are printed one at a time.
Output is line buffered if this appears one second after the last line.