The OP asked "How does next label work?" and provided an edge case example to emphasize his problem.

Well that's perfectly normal ...and a well documented behavior is hardly "golf".

And BTW Basic BLOCKs with redo at the end

{ 
  ... ; 
  redo if COND; 
}
[download]

are a far saner way to realize do-while-loops.

do {
  ...
} 
while (COND)
[download]

EDIT:

I’m of the opinion that especially those coworkers which can't use perldoc -f should be restricted to clean the coffee maker. ;-)