Aha, so you mean I should have only ifs and not if-elseif.

Note that there is a potential pitfall of if (...) {...} if (...) {...} instead of if (...) { if (...) { ... } ... } - it's easy to make a mistake with the logic:

if ( /^[brc]at$/ ) {  # match "bat", "rat", or "cat"
    print "It's an animal\n";
    if ( /^b/ ) {
        print "... and it flies!\n";
    }
}
[download]

This is not the same as:

if ( /^[brc]at$/ ) {
    print "It's an animal\n";
}
if ( /^b/ ) {
    print "... and it flies!\n";
}
[download]

Because the latter will match any word that starts with b.

Of course, in this case, it seems obvious that if the numbers are equal, they will also be within +/- 1 of each other, but mistakes like the above can happen if you don't think the logic through (and test it!).

Update: For example, consider the special case of Inf that syphilis suggested, which I've now added as a test case to my other post.