It leaks memory.

use Devel::Peek;

sub foo {
    my $_recur;
    $_recur = sub {
        $_recur->();
    };

    my $x;

    Dump($x);
    Dump($_recur,0);
}

foo();
[download]

SV = NULL(0x0) at 0x182a4d4
  REFCNT = 1
  FLAGS = (PADMY)
SV = RV(0x182a1e0) at 0x182a1d4
  REFCNT = 2
  FLAGS = (PADMY,ROK)
  RV = 0x2381a4
[download]

Compare the refcount of $x (which will get freed) to $_recur's (which won't).

The problem is that the sub referenced by $_recur captures $_recur, and thus you have a memory loop. If you wanted a lexical, you would need to use

sub foo {
    my $shared_with_all_recur = ...;

    my $_recur;
    $_recur = sub {
        ...
        $_recur->(...);
        ...
    };

    $_recur->(...);

    undef $_recur;
}
[download]

This is a lot messier.