I just found the real functions I was looking for readonly in Scalar::Util. I haven't figured out yet why calling it indirectly via the is_const function doesn't detect that undef is a constant, as perl dies with an error when trying to modify the undef constant, but at least the readonly is easier to read and comprehend. If anyone can explain the undetect undef constant in is_const I'd be greatful.

use Scalar::Util 'readonly';

sub is_const { readonly($_[0]); }

print "undef is readonly\n" if readonly(undef);
print "undef not detected in is_const\n" unless is_const(undef);
for $x (4, "a", undef)
  { print "$x ";$x="VAR" unless readonly($x); print $x,"\n"; }
for $x (4, "a", undef)
  { print "$x ";$x="VAR" unless is_const($x); print $x,"\n"; }
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