It looks as if undef does the job on my system (Linux ... 2.6.20-16-generic #2 SMP ... i686).

system "ps axl | head -1";
my @x; $x[1_000_000] = undef;
system "ps axl | grep $$";
undef @x;
system "ps axl | grep $$";
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VSZ goes from 9040 before the undef to 5132 after.