FWIW, creating an explicit scope and dealing with only a reference to the data does not help, still uses 2GB, with and without usemymalloc.

use GTop qw();
my $gtop = GTop->new;
my $before = $gtop->proc_mem($$)->size;

my $mem;
{
    $mem = \("A" x 1024 ** 3);
}

my $after = $gtop->proc_mem($$)->size;

print GTop::size_string($after - $before);
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