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

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

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is equivalent to 
Y: for my $y ( ... ) {
   X: for my $x ( ... ) {
      next X;
   }
}

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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;
   }
}

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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;
}

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is equivalent to 

for ( @a ) {
   say;
}

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In reply to Re: Question about loops and breaking out of them properly by ikegami
in thread Question about loops and breaking out of them properly by unmatched

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