$/ is (almost) your friend:

local $/;
$/ = ".";

open FH, "./testfile";
open OH, "> ./outputfile";

while (<FH>) {
    s/^(\W*)(\w)/$1\u$2/;
    print OH $_;
}

close FH;
close OH;
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And use this as testfile (make your own spelling errors ;):

this is a test sentence.  this is another one.  we want to test wheter
+ or not this will
make these sentences uppercase.
    another paratraph.  and another sentence.  An upercase one.

a differetn kind of break.

thisone is kind of difficult: www.goof.com.
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note that it doesn't handle the domain name very gracefully, as it will not handle any other instances of embedding periods within sentences as non-periods. However, I can't really think of many:

1. domain names and URLs (and some emails: frank.crist@whatever).
2. decimal numbers.
3. dotted lists, such as this one.
4. elipses (...)

It also doesn't take into account quoted text:
He wondered, "is this in context? possibly."

So, possibly more clever than useful. If the input doesn't follow strict(ish) rules of grammar, you're hosed.