The order is not guaranteed, although the current behaviour happens to be that the reference count decrement is in reverse order of execution of the corresponding 'my' expressions. For example:

sub DESTROY { print "$_[0][0] destroyed\n" }

{
    my $x1 = bless [ "x1" ];
    goto L2;
  L1:
    my $x2 = bless [ "x2" ];
    goto L3;
  L2:
    my $x3 = bless [ "x3" ];
    goto L1;
  L3:
}
__END__
x2 destroyed
x3 destroyed
x1 destroyed
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Dave.