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 however.