Answer #1: You create them with eval:

sub create_subroutine_to_return_constant {
  my ($name, $constant) = @_;
  my $code = "sub $name { return $constant }";
  eval $code;
}
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Then you access them the same as any other subroutine:

create_subroutine_to_return_constant('PI', 3);
$z = PI();      # returns 3
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