# $rgx3 is untested, but provided for example.
# see note below

my $rgx3 = qr/\G        # Global Anchor
            (?<=\.\ ) # Pattern for start of sentence
            (         # $1 
            (\ +)?    # Extra leading spaces ?  
            (.*?\.)   # $2 - the sentence
             )/x;     # end $1 and regex