FYI, the reason nothing is released in the first snippet is:

{
    my $foo = 'X' x 100000000;       <-- a lexical var $foo
    getc;
}
undef $foo;                          <-- the package var $foo was
my $foo = "";                            already undef so does nothing
getc;
[download]

You would have wanted

{
    my $foo = 'X' x 100000000;
    getc;
    undef $foo;
}
getc;
[download]