Prevent leakage of lexical hints

This module works around two historical bugs in Perl's handling of the '%^H' (lexical hints) variable. One bug causes lexical state in one file to leak into another that is 'require'd/'use'd/'do'ed from it. This bug, [perl The second bug causes lexical state (normally a blank '%^H' once the first bug is fixed) to leak outwards from '', if it is automatically loaded during Unicode regular expression matching, into whatever source is compiling at the time of the regexp match. This bug, [perl #73174], was present from Perl 5.8.7 up to Perl 5.11.5, fixed in Perl 5.12.0. Both of these bugs seriously damage the usability of any module relying on '%^H' for lexical scoping, on the affected Perl versions. It is in practice essential to work around these bugs when using such modules. On versions of Perl that require such a workaround, this module globally changes the behaviour of 'require', including 'use' and the implicit 'require' performed in Unicode regular expression matching, and of 'do', so that they no longer exhibit these bugs. The workaround supplied by this module takes effect the first time its 'import' method is called. Typically this will be done by means of a 'use' statement. This should be done as early as possible, because it only affects 'require'/'use'/'do' statements that are compiled after the workaround goes into effect. For 'use' statements, and 'require' and 'do' statements that are executed immediately and only once, it suffices to invoke the workaround when loading the first module that will set up vulnerable lexical state. Delayed-action 'require' and 'do' statements, however, are more troublesome, and can require the workaround to be loaded much earlier. Ultimately, an affected Perl program may need to load the workaround as very nearly its first action. Invoking this module multiple times, from multiple modules, is not a problem: the workaround is only applied once, and applies to everything subsequently compiled. This module is implemented in XS, with a pure Perl backup version for systems that can't handle XS modules. The XS version has a better chance of playing nicely with other modules that modify 'require' or 'do' handling. The pure Perl version can't work at all on some Perl versions; users of those versions must use the XS. On all Perl versions suffering the underlying hint leakage bug, pure Perl hooking of 'require' breaks the use of 'require' without an explicit parameter (implicitly using '$_').

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