What BrowserUk said.
I would look at it slightly differently. If we are talking about a 'class' constant, i.e., something that is invariant over all class and object methods and all 'free' functions of the class, make it a constant as soon as possible, computed or otherwise. If it's an 'object' constant, i.e., something invariant only over the lifetime of a given object, then, of course, it must be somehow created or defined in the constructor: how else? As an example of the first possibility:

package Foo::Bar;
...
use constant RX => qr{ hello }xms;
...
sub new { ... }
...
sub method {
  my $self = shift;
  ...
  if ($self->{bar} =~ RX) { ... }
  ...
  }
...
1;
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