I prefer the first: It shows that you're making a scope for $x and groups the code. I don't care for the others because there's a little extra mental overhead:

• With the BLOCK: version, I start looking for next or goto elsewhere. When I don't see it, I wonder why the label is there.
• The do { ... }; version is the least objectionable, but I do have to look at the end of the loop to see if there's a looping condition, etc.
• The if (1) version is particularly ugly to me. It looks like something is (potentially) optional, that someone hardcoded for the moment, and I wonder in what circumstances you would use a condition.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

Update: Added code tags and changed "I wonder what the condition is where you'd put in a condition" to "I wonder in what circumstances you would use a condition" for clarity. (Thanks, AM)