Restricting a Method to Class-Only o Instance-Only

use Carp qw(croak);

sub instance_only {

ref(my $self = shift ) or croak "instance variable needed";

... use $self as the instance ...

}

sub class_only {

ref(my $self = shift) and croak "class name needed";

... use $class as the class..

}

instance variable needed at their_code line 1234

告诉用户引起问题的行号

croak->die

crap->warn

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