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

# and then...
debug "x=", $x, " y=", $y;
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Update: and sometimes also a -f variant:

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

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