How about doing it via the guts? Search also perlapi for SvIOKp.

# author: bliako
# for: https://perlmonks.org/?node_id=11114538
# 22/03/2020

use Inline 'C';

my $x;
$x = "123"; flags($x);
$x= 123; flags($x);
$x= 123.123; flags($x);

__END__
__C__
void flags(SV *sv){
    printf("int=%d, double=%d, string=%d\n",
        SvIOKp(sv)>>12 & 1,
        SvNOKp(sv)>>13 & 1,
        SvPOKp(sv)>>14 & 1
    );
}
[download]

bw, bliako