where did you assigned value to $pid ? :-)
You can also miss some data, for example @cmd can handle TERM signal and print partial result. @cmd also can block/not handle TERM signal (or in some case, cannot be KILLed).

Sample: 

use strict;
use warnings;

use IO::Pipe;
use constant TIMEOUT => 5;

my @cmd = ();
my $pipe = new IO::Pipe;

if (my $pid = fork) {
    $pipe->reader;

    $SIG{ALRM} = sub {
        kill TERM => $pid;
        $SIG{ALRM} = sub {
            kill KILL => $pid;
            $SIG{ALRM} = sub { close $pipe; };
            alarm TIMEOUT;
        };
        alarm TIMEOUT;
    };
    $SIG{CHLD} = sub {
        while (my $wait = wait) {
            alarm 0 if $wait == $pid;
        }
    };

    # alarm here if you want force child to quit after TIMEOUT
    # alarm TIMEOUT;
    while (my $from_child = <$pipe>) {
        # alarm here if you want to interrupt child after inactivity
        # (no output lines)
        # alarm TIMEOUT;
        # ...
    };
    alarm (0);

    close $pipe;
} elsif (defined $pid) {
    # Child

    $pipe->writer;

    open STDOUT, '>&', $pipe;

    exec @cmd;
} else {
    die "Fork failed: $!\n";
}

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In reply to Re^2: fork - alarm - output by happy.barney
in thread fork - alarm - output by chart3399

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