Perl module to prevent leakage of lexical hints

Lexical::SealRequireHints 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 required/used from it. This bug, [perl #68590], was
present from Perl 5.6 up to Perl 5.10, fixed in Perl 5.11.0. The second bug
causes lexical state (normally a blank %^H once the first bug is fixed) to leak
outwards from utf8.pm, 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, so that it no longer exhibits these bugs.