You're constantly setting (and re-setting) a global variable called %data. Since you give a reference to this back to your TIEHASH function, any time you write into (or over) %data, that will appear in every single instance of this TIEHASH you have.

Now, a good solution would be to not return a reference, but return a hash. Then, take a reference to the copied hash (which is now not called %data), and you're fine.

A better solution would be this:

# Untested change!!!
sub _read { # still need to deal with race conditions
   my $file = shift;

   return undef unless -f $file;

   our %data;
   unless (my $ret = do $file) {
      warn "couldn't parse $file: $@" if $@;
      warn "couldn't do $file: $!"    unless defined $ret;
   }

   return \%data;
}
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