my @INPUT = (... many lines of text ...);
my @COMMAND = qw(tr a-zA-Z A-Za-z); # swap lower/upper case
my @RESULT; # this is where the result will go...
if (open RESULT, "-|") { # original process
  @RESULT = <RESULT>;
} else { # child
  if (open STDIN, "-|") { # child
    exec @COMMAND;
    die "Cannot exec: $!";
  } else { # grandchild
    print @INPUT;
    exit 0;
  }
}