__PACKAGE__ is only special when used as a constant; putting it in a string (or a regex) makes it not a constant. While you could do ... =~ quotemeta(__PACKAGE__), this is not the issue. If you want to see if the first argument to your function is an object, I would suggest:

sub something {
  if (eval { $_[0]->isa(__PACKAGE__) }) {
    # object call
  }
  else {
    # function call
  }
}
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