Use our instead of my and your code will work as you expect.

C:\test\DBMNested>p1
[0] Perl> sub getChecksum($) {
        my $in = shift;
        our $check = 0;
        $in =~ m/(?:(.)(?{$check += ord($1);}))*^/;
        return $check;
};;
print getChecksum( 'foo' );;
324

print getChecksum( 'foo' );;
324

print getChecksum( 'foobar' );;
633
[download]

Though its a very slow way to achieve your goal. Corion's unpack tip is the way to go.