you initialize my $FIXED_STRING = 'fixed_string'; only AFTER calling the sub.

Not really a closure problem, could happen with package vars too.

One best practice to avoid such problems is to use a "generator sub" with the whole closure-context like in your block and returning \&do_something

update

like:

sub generate {

    my $FIXED_STRING = 'fixed_string';

    my %persistent;

    return sub {
   
         ...   
}
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