Better late than never: here's my take on the situation.

$test = "This is a nice weather. He is a nice guy. We have some nice f
+ood in our dinner.";

$test =~ s/(?:.*?)(\bnice\b[^.]*\.\s*)/A $1/g;

print $test;
[download]

The (?: allows grouping without backreferencing, thus making the regex a little more efficient. The [^.] after the word "nice" forces it to match up to, but excluding the period. This is much more efficient than the dot star ".*" because of the way Perl's regex engine works. Unfortunately, I could not figure out a clean way of getting rid of the first dot star.

Cheers,
Ovid