A regex that catches common cases is fairly easy. However handling nasty nested cases with full stops that are not adjacent to a quote heads into CSV teritory. Here's the simple case with an example of not handling something a little nastier:

use warnings;
use strict;

my $str = do {local $/ = ''; <DATA>;};
print join "^", split /(?<!['"])\.(?!['"])/, $str;

__DATA__
this is some text.
A period (".") usually terminates a statement.
But not if it's quoted.
Regardless of whether or not single quotes, '.', are used.
"Quoted sentences with a . in the middle." could be harder to manage h
+owever.
[download]

Generates:

this is some text^
A period (".") usually terminates a statement^
But not if it's quoted^
Regardless of whether or not single quotes, '.', are used^
"Quoted sentences with a ^ in the middle." could be harder to manage h
+owever
[download]