It is a bit odd. Why not let the exception propagate?

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

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