a thing I like to do is put a leading underscore on methods that should not be called directly e.g.

...

sub external {
  ...
  _internal();
  ...
}

sub _internal {
}
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this is just a hint to the class user, of course. I also do the same for function-style modules, I put required methods in EXPORT, internal functions in EXPORT_OK, and provide :std and :test tags in EXPORT_TAGS that are EXPORT and EXPORT_OK e.g.

@EXPORT = qw(external interface);
@EXPORT_OK = qw(_internal _interface);
%EXPORT_TAGS = (STD  => \@EXPORT,
                TEST => \@EXPORT_OK);
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