i've made some modifications to your code to make it a bit safer, and to capture more output:

== your initial code didn't check for a valid set of commands, so someone could have run malicious code and sent you an email after the fact. i created a set of valid commands to prevent this
== i added error checking all around, including creation of mail object, io handles, etc.
== i seperated the output to STDERR and STDOUT into different streams, for better reporting
== i captured the exit code and process id, just to be complete
== i sent output to the screen as well as in an email.

see below.

#!/usr/local/bin/perl -w
use strict;
$|++;

use constant EMAIL => 'you@yourdomain.com';
use constant SERVER => 'mail.yourdomain.com';

use IO::Handle;
use IPC::Open3;
use Net::SMTP;
use Mail::Mailer;

# create list of valid commands (put your commands in here!)
#   stop malicious usage!!!
my $valid_commands = "(backup_nt|backup_unix|backup_linux)";

my $n = "\n";

# get command from command line, display usage if error
my $cmd = shift 
    || die("usage: $0 $valid_commands [opts]\n");

# verify command is valid
die "invalid command: $cmd"
    unless($cmd =~ /$valid_commands/);

my $cmdopts = join ' ', @ARGV;

my $mail = Net::SMTP->new( SERVER ) 
    || die("cannot create mail server! $!");

$mail->to( EMAIL );
$mail->mail( 'root@yourdomain.com' );
$mail->data();

my (@output, $test, $pid, $exitcode);

# create local io handles, fail if open fails
my $OUTPUT = new IO::Handle;    # create handle for STDOUT
my $OUTERR = new IO::Handle;    # create handle for STDERR
my $IN = new IO::Handle;        # create handle for STDIN
open( $OUTPUT, ">&STDOUT" ) or     # duplicate STDOUT
    die "Can't dup STDOUT to OUTPUT: $!$n";
open( $OUTERR, ">&STDERR" ) or    # duplicate STDERR
    die "Can't dup STDERR to OUTERR: $!$n";
open( $IN, "<&STDIN" ) or          # duplicate STDIN
    die "Can't dup STDIN to IN: $!$n";

$test = eval                       # execute $cmd, redirecting to new 
+handles,
{                                  #   and capture the process id
    $pid = open3($IN, $OUTPUT, $OUTERR, "$cmd $cmdopts") or die($!);
    my $junk = waitpid(-1,0);      # don't care about return val
};
$exitcode = $? >>8;                # $? is exitcode of $s ( from waitp
+id() )

# format output for mailing
push @output, " -> Result:" . $n;
push @output, $_ while(<$OUTPUT>);
push @output, " -> Errors:" . $n;
push @output, $_ while(<$OUTERR>);
push @output, " -> Exit Code: " . $exitcode . $n;
push @output, " -> Process ID: " . $pid . $n;

close $OUTPUT;                     # close OUTPUT handle
close $OUTERR;                     # close OUTERR handle
close $IN;                         # close IN handle

# send error if eval failed, otherwise send output
if ($@) {
    $mail->datasend("command '$cmd $cmdopts' could not be executed: $@
+\n");
    print "command '$cmd $cmdopts' could not be executed: $@\n";
} else {
    $mail->datasend( 
        " -> Command: $cmd $cmdopts" , $n,
        join('', @output), $n,
    );
    print(
        " -> Command: $cmd $cmdopts" , $n,
        join('', @output), $n,
    );
}

$mail->datasend;
$mail->quit;
exit 0;
[download]

~Particle