Here's the same code, reformatted into context:

# Beware of using input from the user in $cmd.
my $cmd = 'for f in a b c; do date; sleep 1; done';

while (my $conn = $sock->accept()) {
   open(PROG, "$cmd|" or die $!;
   while (<PROG>) { $conn->print($_); }
   close(PROG);
}
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If you need to send stuff to $cmd's STDIN, check out the IPC::Open2 and IPC::Open3 modules (included with perl) as an alternative to open ...|.