Hey,
You really should use Taint mode when dealing with system calls in CGI (and in general, too).

OeufMayo - live from HAL2001

Can't argue with that! Here's a more secure version.

#!/usr/bin/perl -wT
 
use strict;
 
# debug mode
BEGIN {
#   use CGI::Carp qw(carpout fatalsToBrowser);
#   &carpout (\*STDOUT);
}
 
# declare global vars
our ($hour, $minute, $day, $meridiem);
 
use CGI;
my $q = new CGI;
print $q->header();
print "<center>\n";
&print_at_form;
&create_at_job if $q->param('Submit');
 
### Subroutines ###
sub print_at_form {
    print $q->startform();
    print "Hour";
    print $q->popup_menu(-name=>'hour', -values=>[1..12]);
    print "Minutes";
    print $q->popup_menu(-name=>'minute', -values=>[qw/00 05 10 15 20 
+25 30 35 40 45 50 55/]);
    print "Day";
    print $q->popup_menu(-name=>'day', -values=>[qw/Today Mon Tue Wed 
+Thu Fri Sat Sun/]);
    print $q->radio_group(-name=>'meridiem', -values=>[qw/am pm/]);
    print $q->submit(-name=>'Submit');
    print $q->endform();
}
 
sub _untaint {
    $ENV{"PATH"} = "";
    if ($q->param('hour') =~ /^(1|2|3|4|5|6|7|8|9|10|11|12)$/) { $hour
+ = $1; } else { die "Can't untaint 'hour'"}
    if ($q->param('minute') =~ /^(00|05|10|15|20|25|30|35|40|45|50|55)
+$/) { $minute = $1; } else { die "Can't untaint 'minute'"}
    if ($q->param('day') =~ /^(Today|Mon|Tue|Wed|Thu|Fri|Sat|Sun|)$/) 
+{ $day = $1; } else { die "Can't untaint 'day'"}
    if ($q->param('meridiem') =~ /^(am|pm)$/) { $meridiem = $1; } else
+ { die "Can't untaint 'meridiem'"}
}
 
sub create_at_job {
    _untaint;
    my $string = "/usr/bin/at $hour:$minute $meridiem $day";
    my $pid = open(ATJOB, "| $string") or die "Couldn't fork: $!\n";
    print ATJOB "touch /foo/bar/some_file\n";
    close(ATJOB);
    print "Your job has been scheduled for <b>$hour:$minute $meridiem 
+$day</b>";
}
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