It will just call the destroy method and return. For example this:

sub Foo::DESTROY { my $self = shift; print "destroying $self\n" };
my $foo = bless {}, "Foo";
$foo->DESTROY;
print "foo is $foo\n";
[download]

destroying Foo=HASH(0x4759c)
foo is Foo=HASH(0x4759c)
destroying Foo=HASH(0x4759c)
[download]

The first "destroying" comes from the explicit call to the method. The second by the method being called before the object is garbage collected.