Letting a variable fall out of scope doesn't free up all its memory. The memory used for the members of an aggregate (array or hash) are generally freed, however the memory used for the structure of the variable (including scalars) aren't freed--they are instead saved in case the scope is re-entered. Notice:

use Devel::Size qw(size);
$foo = 2;
while ($foo--) {
  {
    my $bar;
    print Devel::Size::size(\$bar), " ", length($bar), "\n";
    $bar = " " x 10000000;
  }
}
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prints

12 0
10000025 0
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not

12 0
12 0
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which is what you'd expect if the memory was actually freed on scope exit.