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    can(METHOD)

    can checks to see if its object has a method called METHOD. If it 
+does
    then a reference to the sub is returned; if it does not then undef
+ is
    returned.

    package A;

...
        my $self = shift;
        return $self->can("_write_file");
    }

    package A::B;
    use A;
...
        my($path, $val, $start) = @_;
        # ...
    }