while (1) {
    # get the date
    if ((localtime)[6] eq 4) {
        # do processing
        ...
    } else {
        sleep 86400;
    }
}

##</code><code>##

my $interrupted = 0;
$SIG{INT} = sub { $interrupted = 1; &save_file; die; };
while (!$interrupted) {
    # get the date
    if ((localtime)[6] eq 4) {
        # do processing
        ...
    } else {
        sleep 86400;
    }
}