There are lots of ways, not all equivalent. Unicode, for instance, makes the question a lot more complicated, as does locales.

For plain declarative sentences in ASCII, like you are parsing for,

while(<>){
    if(/\.\s+([A-Z]+)/){
        print "$1 is capitalized\n";
    }
}
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The perl functions uc, lc, ucfirst, and lcfirst are handy for this kind of comparison. Accepting the sentence matching of your example for simplicity,

if ( /\.\s+(\w+)/ and $1 eq ucfirst($1) ) {
    print "That'un's Ok", $/;
}
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To solve the sentence after linebreak problem, you can either slurp the entire file by local $/=undef; and m//s, or else match terminal periods and keep state in some variable for the next line.

After Compline,
Zaxo

1. The first sentence, which has no preceding punctuation-whitespace.
2. A "regular" sentence that has the punctuation-whitespace-word sequence.
3. A regular sentence that happens to start on a new line.

#!/usr/bin/perl -w
use strict;

@ARGV = '/Perl/LearningPerl/Test';
local $/ = '';

for(<>) {   
    if(/^([a-z]+)/) {
        print "\'$1\' is not capitalized\n";
    }
    while(/[:.?!]\s+([a-z]\w*)/g){
        print "\'$1\' is not capitalized\n";
    }
}
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Hmmmm, this will get all non-capitalized words at the beginning of each sentance except the very first sentance and store them into an array:

my $s = 'hello world! how are you? whoops, forgot.';

my @no_caps = $s =~ /[.?!]\s*((?![A-Z])[a-z]\w+)/g;
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$s =~ s/^((?![A-Z])[a-z]\w+)/ucfirst$1/e;
$s =~ s/([.?!]\s*)((?![A-Z])[a-z]\w+)/$1.ucfirst$2/eg;
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use strict;

@ARGV = '/Perl/LearningPerl/Test';

while(<>){
   if (/^((?![A-Z])[a-z]\w+)/) {
      print "$1 is not capitalized\n";
   }
   while (/([.?!]\s*)((?![A-Z])[a-z]\w+)/g) {
      print "$2 is not capitalized\n";
   }
}
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And it is still broken, as newlines are the monkeywrench in this machine. Taking Zaxo's suggestion of slurping the entire file into a scalar will fix that (the error, not the ugliness):

my $file = do {local $/; <>};

if ($file =~ /^((?![A-Z])[a-z]\w+)/) {
    print "$1 is not capitalized\n";
}
while ($file =~ /([.?!]\s*)((?![A-Z])[a-z]\w+)/g) {
    print "$2 is not capitalized\n";
}
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jeffa

Hmmmm . . .not sure how efficient this is, but this is how I'd do it.

$paragraph = shift;
@sentences = split /[.!?]/;
foreach (@sentences) {
    last unless /\w/; #Make sure that we have words
    print unless /^\s*[A-Z]/;
}
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Light a man a fire, he's warm for a day. Catch a man on fire, and he's warm for the rest of his life. - Terry Pratchet

Update:Fixed stupid bug.

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