It seems to raise a perfectly valid exception for me:

sub round {
    my $self = shift;
    my $rnd = $self->{precision};
    my $number = shift;
    return(sprintf("%0.${rnd}f", $number));
};;

print round( {precision=>3}, 1.23456 );;
1.235

print round( {precision=>3}, 1 );;
1.000

print round( {precision=>3} );;
Use of uninitialized value $number in sprintf at ...
0.000
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(Though quite why you'd use a module for this is beyond me?)