# phpman > perldoc > Internals

## NAME
    Internals - Reserved special namespace for internals related functions

## SYNOPSIS
        $is_ro= [Internals::SvREADONLY](https://www.chedong.com/phpMan.php/perldoc/Internals%3A%3ASvREADONLY/markdown)($x)
        $refcnt= [Internals::SvREFCNT](https://www.chedong.com/phpMan.php/perldoc/Internals%3A%3ASvREFCNT/markdown)($x)
        hv_clear_placeholders(%hash);

## DESCRIPTION
    The Internals namespace is used by the core Perl development team to expose certain low level
    internals routines for testing and other purposes.

    In theory these routines were not and are not intended to be used outside of the perl core, and
    are subject to change and removal at any time.

    In practice people have come to depend on these over the years, despite being historically
    undocumented, so we will provide some level of forward compatibility for some time. Nevertheless
    you can assume that any routine documented here is experimental or deprecated and you should
    find alternatives to their use.

  FUNCTIONS
    SvREFCNT(THING [, $value])
        Historically Perl has been a refcounted language. This means that each variable tracks how
        many things reference it, and when the variable is no longer referenced it will
        automatically free itself. In theory Perl code should not have to care about this, and in a
        future version Perl might change to some other strategy, although in practice this is
        unlikely.

        This function allows one to violate the abstraction of variables and get or set the refcount
        of a variable, and in generally is really only useful in code that is testing refcount
        behavior.

        *NOTE* You are strongly discouraged from using this function in non-test code and especially
        discouraged from using the set form of this function. The results of doing so may result in
        segmentation faults or other undefined behavior.

    SvREADONLY(THING, [, $value])
        Set or get whether a variable is readonly or not. Exactly what the readonly flag means
        depend on the type of the variable affected and the version of perl used.

        You are strongly discouraged from using this function directly. It is used by various core
        modules, like "[Hash::Util](https://www.chedong.com/phpMan.php/perldoc/Hash%3A%3AUtil/markdown)", and the "constant" pragma to implement higher-level behavior
        which should be used instead.

        See the core implementation for the exact meaning of the readonly flag for each internal
        variable type.

### hv_clear_placeholders
        Clear any placeholders from a locked hash. Should not be used directly. You should use the
        wrapper functions provided by [Hash::Util](https://www.chedong.com/phpMan.php/perldoc/Hash%3A%3AUtil/markdown) instead. As of 5.25 also available as "
        [Hash::Util::_clear_placeholders](https://www.chedong.com/phpMan.php/perldoc/Hash%3A%3AUtil%3A%3Aclearplaceholders/markdown)(%hash) "

## AUTHOR
    Perl core development team.

## SEE ALSO
    perlguts [Hash::Util](https://www.chedong.com/phpMan.php/perldoc/Hash%3A%3AUtil/markdown) constant universal.c