in summary:

use Cwd;
sub TIESCALAR {bless {} => shift}
sub FETCH     {cwd}
sub STORE     {defined $_[1] and (chdir $_[1] or die $!)}
our $cwd;
tie $cwd => __PACKAGE__;
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that defined $_[1] is necessary becuase local seems to undef the variable at some point (otherwise resulting in a chdir to $HOME).