sub debug {
  warn join '', map $_ // '<undef>', @_, "\n";
}

# and then...
debug "x=", $x, " y=", $y;


##</code><code>##

sub debugf {
  my $fmt = shift;
  warn sprintf($fmt, map $_ // '<undef>', @_) . "\n";
}

debugf "x=%s y=%s", $x, $y; # <-- always using %s!