if ($pounds == 0.0 and $grams == 0.0) {
    ...
}

##</code><code>##

D:\>perl -E "say '' == 0.0 ? 'Equals' : 'Does not equal'"
Equals

D:\>perl -E "say '0' == 0.0 ? 'Equals' : 'Does not equal'"
Equals

D:\>perl -E "say '0.0' == 0.0 ? 'Equals' : 'Does not equal'"
Equals

D:\>perl -E "say '123' == 0.0 ? 'Equals' : 'Does not equal'"
Does not equal

D:\>perl -E "say '123.4' == 0.0 ? 'Equals' : 'Does not equal'"
Does not equal

D:\>