I thought next is just another way to say goto

Well, it's not. You can see that next and goto compile to different ops internally:

$ perl -MO=Concise -e 'X: next X'
4  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(X: main 1 -e:1) v:{ ->3
3     <"> next("X") v ->4
-e syntax OK
$ perl -MO=Concise -e 'X: goto X'
4  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(X: main 1 -e:1) v:{ ->3
3     <"> goto("X") v ->4
-e syntax OK
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Another difference is this warning here:

$ perl -wE 'sub f() { next X }; X: for (1) { f }'
Exiting subroutine via next at -e line 1.
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which you don't get with goto. And of course the fact that the code above terminates, but doesn't if you replace 'next' with 'goto'.