I reduced it to 1 GB and was still able to reproduce it.

But the effect disappeared, after I changed the logic to avoid temporary data on the RHS.

use v5.12;
use warnings;
use Devel::Size qw(total_size);

my $x = 'a';
$x x= 2**30;  # 1GiB
print total_size($x) . "\n";
sleep 60;
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Seems like the allocated extra space for your 'a' x (2**32) wasn't released.

update

see also Re^2: Memory usage double expected (run-time)