open(TMP, "| cmd");

open(TMP, "cmd |");

Worked example

Write a Perl script to take a string as first argument, then read standard input and copy all lines containing that string to standard output.
Solution: This is a job for fgrep. The ARGV array contains the string as its first element. Construct the command fgrep with the string as its argument (note that since the string may contain spaces, you need to enclose the argument in single quotes). Open a pipe into the command, and loop through the lines of standard input, not forgetting to close the file after use. The output of fgrep by default goes to standard output, so a second pipe is unnecessary. Closing the filehandle is required even though no named file is used.

# Set the value of variable string to the first argument
$string=$ARGV[0];

# Create a shell command to search for that string
$command="fgrep '$string'";

# Open a pipe to that command
open(FGREP, "| $command");

# Repeatedly real lines from standard input ...
while (<STDIN>) {
  # ... and send them to the pipe
  printf FGREP $_;
  }

# Finally, close the pipe
close(FGREP);