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

##</code><code>##

my ($pretty_number) = eval { Phone::prettify($number) }
   or do { ...insert message $@ into web page output...
           ...exit/return/next/etc...
         };