sub remove_sign_on_zero {
    my $num_str = shift;
    if ($num_str =~ /^-([0\.]+)$/) {
        return " $1";
    } 
    else {
        return $num_str;
    }
}

my $num = remove_sign_on_zero( sprintf('%7.5f', $example) );

##</code><code>##

sub remove_sign_on_zero { $_[0]=~/^-([0\.]+)$/ ? " $1" : $_[0] }