$x = '' unless defined($x)

##</code><code>##

my $x = '';

##</code><code>##

defined($x) or confess;

##</code><code>##

mysub($user_data)
{
  $user_$data->{invoice_number} ...
}
mysub({age => $user_age, inovice_number = $invoice_number}) # TYPO!

##</code><code>##

mysub($user_data)
{
  defined($user_$data->{invoice_number}) or confess;
  $user_$data->{invoice_number} ...
}
mysub({age => $user_age, inovice_number = $invoice_number}) # TYPO!