If the undef has a significant meaning, I define it explicitly to make the code as clear as possible. Otherwise, I spend my keystrokes elsewhere. For example:

my $found = undef;
foreach my $candidate (@candidates) {
  if ( good($candidate) ) { 
    $found = $candidate; last;
  }
}
if (defined $found) {
  print "Found a good candidate: $found\n";
}
[download]