Prototypes are a horrible idea. Still if you want to make that mistake you can by predeclaring the function so that inside the function it knows the prototype:

sub check_ip();
sub check_ip() {
    eval{
        my $ip_test = new Net::IP(`curl whatismyip.org`);
        return $ip_test->ip();
        };
    if ($@){
        sleep(300);
        check_ip();
    };
}
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