if ( @ARGV ) {
    do_something( shift );
}
elsif ( -M $file > 7 ) {
    do_something_else();
}
else {
    warn "Nothing to do ($file is recent, and you gave no arg to work on)\n";
    warn "Usage: $0 [optional_arg_that_does_what_exactly]\n";
}
# default exit status will be "0" (success)