package Phone;
sub prettify {
    local ($_) = @_;
    return eval {
        /^$/ and die "No number\n";
        # do something to modify $_ here
        $_;
    };
}

##</code><code>##

my $pretty_number = Phone::prettify($number)
if (not defined($pretty_number)) {
    # e.g. insert message $@ into web page output
}