I've been closing over hashes for years, for subs that need some persistent store, but not wanting to invoke memoize.

Below I've closed over a scalar and a hash. I get a warning of:

Use of uninitialized value $FIXED_STRING in hash element at ...
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With hashes, this just works, but apparently the hash is empty, and that's fine. But the scalar value within the sub is undef.

What am I missing?

What is the easiest way to fix this, while still using a scalar to store a string, that is later used as a hash key?

#!/usr/bin/env perl

# demo of closure around sub, with persistent string

use strict;
use warnings;

for my $q ('a'..'g') {
    do_something($q);
    print "Done computing\n"
}

{
    my $FIXED_STRING = 'fixed_string';
    my %persistent;

    sub do_something {
        my $x = $_[0];

        $persistent{$x}{$FIXED_STRING} = rand;

        END {
            for my $k (keys %persistent) {
                print "$k: $persistent{$k}{$FIXED_STRING}\n";
            }
        }
    }
}
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