Sorry for the late answer, found your question by chance via Google.
You probably stumbled over one of the many bugs in the old BigFloat package. There is a newer, much improved version on CPAN, and you should give it a try. It also has a bsqrt() function, that actually works ;)

    use Math::BigFloat v1.22;

    $x = Math::BigFloat->new('1234.567');

    print "square root of $x is ",$x->bsqrt(),"\n";
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HTH Tels

If you have more questions, feel free to contact me at tels at bloodgate dot com. Thanxy!