$variable is never assigned a value, so the stringification of $variable when it's used as a keyissues a warning.

$variable => \$variable,
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It was very easy to debug by removing elements from the hash until the error disappeared.

By the way, are you aware of Data::Dumper? It does the same thing as your code.

use Data::Dumper;
$Data::Dumper::Useqq = 1;
my $x = {
   ...
};
print Dumper $x;
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