my  @CMDS = ("ps", "top", "df", "vmstat", "blah");
my  @FILES = ( "/etc/running.cfg", "/home/me/lastchecked.txt", "/var/log/httpd/error_log");

my ($cmd, @ARGS) = split(/\s/, $input);
my $ok;
for(@CMDS) {
	if($cmd eq $_) { $ok=1; }
}
if(!$ok) { die "tried to do something that wasn't cool: $input\n"; }
# then check for files in args
my $ok=0;
my $arg;
foreach $arg (@ARGS)
	if(-e $arg) { 
		# we have a file
		$file=true;
		for(@FILES) {
			if($arg eq $_) {
				$ok=1;
			}
		}
	}
}
if($file && !$ok) { die "tried to access a bad thing: $input\n"; }
# We're pretty much safe to continue