I wanted to be able to tie the original variable while changing the original data.

I see. I remember that when I tried something similar to that with tied filehandles, I got Perl to segfault, so I'm not sure if tie is meant to be used that way.

It's not always possible to replace the monitored variable with a proxy, you just want to attach the magic directly.

That's probably true, but I imagine it wouldn't be too difficult to refactor the code in question from using a hash to a hashref?

Otherwise, if this is just for debugging, you could use refaliasing to replace the target hash with the tied hash? (though to unite you'd probably have to keep a reference to the original hash around)

use warnings;
use strict;
use feature 'refaliasing';
use Data::Dump;
use Tie::Subset::Hash;

my %hash;
@hash{"a".."e"} = 41..45;

dd \%hash;

tie my %hash2, 'Tie::Subset::Hash', \%hash, ['b'];
\%hash = \%hash2;

dd \%hash;

__END__

Aliasing via reference is experimental at foo.pl line 13.
{ a => 41, b => 42, c => 43, d => 44, e => 45 }
{
  # tied Tie::Subset::Hash
  b => 42,
}
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