#!perl -swl
use Math::MPFR qw(:mpfr);

my $fr = Math::MPFR->new();
print $fr;
warn "nan flag not set\n" unless Rmpfr_nanflag_p();

$fr *= 2;
warn "nan flag set\n" if Rmpfr_nanflag_p();
print $fr;
Rmpfr_clear_nanflag(); # Clear nan flag


__END__

Outputs:

@NaN@
nan flag not set
nan flag set
@NaN@