Depending on the properties of the data, one might be better off insisting on whitespace after period to define a full-stop:

$_ = "Next line (2.) contains a url. The url is perlmonks.org. Send th
+em \$20.00.\n";

@sentences = split(/(?<=\.)\s+/);

print "$_\n" for ( @sentences );
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Of course, things like "Mr. Smith", etc. will make it... a little more complicated. (Full stops not followed by space "are left as an excercise for the reader.")