Without a label, next/last/redo affects the closest (innermost containing) loop.

for my $y ( ... ) {
   for my $x ( ... ) {
      next;
   }
}
[download]

is equivalent to

Y: for my $y ( ... ) {
   X: for my $x ( ... ) {
      next X;
   }
}
[download]

To go to the next pass of the outer loop, you can use the following:

Y: for my $y ( ... ) {
   X: for my $x ( ... ) {
      next Y;
   }
}
[download]

In your example, the next is inside an if statement's body (not a loop), which is inside a foreach loop's body. It will go to the next pass of the only/foreach loop.

---

Notes

In your example, the next isn't in the grep "loop". It's in the if statement's body which is executed once the grep is complete.

You should not use next/last/redo in the callback of the grep operator.

Be careful. { ... } as a while statement is a loop. This means that

for ( @a ) {
   {
      last;
   }
   say;
}
[download]

is equivalent to

for ( @a ) {
   say;
}
[download]