It's documented in next

> "It should not be used to exit a grep or map operation."

Tho it's a bit inconsistent, because return clarifies that those grep/map blocks are special, but doesn't discourage the use.

> Returns from a subroutine, eval, do FILE, sort block or regex eval block (but not a grep, map, or do BLOCK block) with the value given in EXPR.

On a side note: I've never thought of using return inside a sort block and can't remember ever seeing it.

Update

So I tried it out in the debugger and next inside a grep block is doing exactly what I expected, it's ignoring grep and jumps to the next iteration of the surrounding loop. IOW

• grep is not considered a loop
• grep's block is breakable

perl -de0

DB<12> for (1..9) {say grep {next if  $_<5;$_%2} $_..9}
579
79
79
9
9
[download]

PS: yes it's a convoluted example.