Why is it bad to put a space before a semicolon?

From the first paragraph of the Description section:

Each programmer will, of course, have his or her own preferences in regards to formatting, but there are some general guidelines that will make your programs easier to read, understand, and maintain.

And from the third paragraph of the Description section:

Regarding aesthetics of code lay out, about the only thing Larry cares strongly about is that the closing curly bracket of a multi-line BLOCK should line up with the keyword that started the construct. Beyond that, he has other preferences that aren't so strong:

It's just a convention among Perl programmers for style. If you don't like it, you can choose to ignore it. I would compare it to Sun's Java Style guidelines or the suggestions for indenting in the K&R C book. Some environments have their own guidelines. These just happen to be the suggestions for Perl.

As Jon Skeet said in a deleted answer to this question:

If you're happy to be inconsistent with what some other people like, then just write in the most readable form for you. If you're likely to be sharing your code with others - and particularly if they'll be contributing code too - then it's worth trying to agree some consistent style.

This is only my opinion, but I also realize that people read code in different ways so "bad' is relative. If you are the only person who ever looks at your code, you can do whatever you like. However, having looked at a lot of Perl code, I've only seen a couple of people put spaces before statement separators.

When you are doing something that is very different from what the rest of the world is doing, the difference stands out to other people because their brain don't see it in the same way it. Conversely, doing things differently makes it harder for you to read other people's code for the same reason: you don't see the visual patterns you expect.

My standard is to avoid visual clutter, and that I should see islands of context. Anything that stands out draws attention, (as you say you want), but I don't need to draw attention to my statement separators because I usually only have one statement per line. Anything that I don't really need to see should fade into the visual background. I don't like semi-colons to stand out. To me, a semicolon is a minor issue, and I want to reduce the number of things my eyes see as distinct groups.

There are times where the punctuation is important, and I want those to stand out, and in that case the semicolon needs to get out of the way. I often do this with the conditional operator, for instance:

my $foo = $boolean ?
            $some_long_value
                  :
            $some_other_value
                  ;

If you are a new coder, typing that damned statement separator might be a big pain in your life, but your pains will change over time. Later on, the style fad you choose to mitigate one pain becomes the pain. You'll get used to the syntax eventually. The better question might be, why don't they already stand out? If you're using a good programmer font that has heavier and bigger punctuation faces, you might have an easier time seeing them.

Even if you decide to do that in your code, I find it odd that people do it in their writing. I never really noticed it before Stackoverflow, but a lot of programmers here put spaces before most punctuation.

It's not a rule, it's one of Larry Wall's style preferences. Style preferences are about what help you and the others who will maintain your code visually absorb information quickly and accurately.

I agree with Larry in this case, and find the space before the semicolon ugly and disruptive to my reading process, but others such as yourself may find the exact opposite. I would, of course, prefer that you use the sort of style I like, but there aren't any laws on the books about it.

Yet.