Here is a little script for linux only that can help to understand what happens to memory:

our @test;

print "nothing : Size = ".size()."\n";
push @test, rand for (1..100000);
print "push : Size = ".size()."\n";

#undef @test;
@test = ();

print "deleted : Size = ".size()."\n";

sub size {
  open DATA, "< /proc/$$/status" or die "Unable to read data : $!\n";
  local $/;
  <DATA> =~ m/^VmSize:\s+(.*?)$/m;
  return $1;
}
[download]

If I use @test = ();, output is

nothing : Size = 2988 kB
push : Size = 5492 kB
deleted : Size = 5492 kB
[download]

and with undef @test;

nothing : Size = 2988 kB
push : Size = 5488 kB
deleted : Size = 4976 kB
[download]

Of course, this example is OS dependant. But you can notice that the first solution does not free any OS memory at all. As FatVamp said, Perl can then reuse the memory. It can be noticed that a few more memory is freed when using undef, but not all that was used.

HTH

--
zejames

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