You can rely on convention, and simply name things with an underscore as the first character of the sub's name (and put in a note to the fact). If anyone gets burned by using the private interface, it's their own fault.

A truly private option would be to stash coderefs in a lexically declared hash.

package Foo;

my %hidden = (
              bar => sub { print "Bar\n"; },
              ...
             );

sub something_public {
  my( $baz, $quux ) = @_;

  return $hidden{bar}->( $baz + 5, reverse $quux );
}
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(disregard the mispost above) In reference to the hash declaration am I able to declare variables contained within the hash like my %hidden = ( bar=>sub { my $foo = shift; print $foo; } ); Instead of using one line of code per entry?

package SomeClass;
use Attribute::Protected;

sub new { return bless {}, shift }

sub foo : Public {
    print "am in public foo()\n";
}
sub bar : Private {
    print "am in private bar()\n";
}
qq[end of package];

package main;

$x = SomeClass->new();
$x->foo();             # fine - foo is public
$x->bar();             # dies - bar is private
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broquaint

Thanks to all the replies, I'll have a look at both techniques mentioned. ceedee