my $x;
$x *= 2;

Use of uninitialized value $x in multiplication (*) at ...

Good analogy. With the mpfr library (as with perl), such an operation does not cause a fatality - but a warning *is* provided (if the program explicitly looks for it) in that the nan flag is set:

#!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@
[download]

Maybe the Math::MPFR/Math::MPC overloading of the arithmetic operators should warn about operations involving NaN's by default (at least if warnings are enabled) ... that would be quite a perlish thing to do!

Cheers,
Rob