Did you look at the docs for Statistics::Distributions? Or the code? If you had, you would see that fprob only accepts positive integer values for the degrees of freedom (as it should).

sub fprob { # Upper probability   F(x,n1,n2)
    my ($n, $m, $x) = @_;
    if (($n<=0) || ((abs($n)-(abs(int($n))))!=0)) {
        die "Invalid n: $n\n"; # first degree of freedom
    }
    if (($m<=0) || ((abs($m)-(abs(int($m))))!=0)) {
        die "Invalid m: $m\n"; # second degree of freedom
    } 
    return precision_string(_subfprob($n, $m, $x));
}
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