my $n =  # choose your rgx number

# skip a new-line beginning with a literal space.

my $rgx1 = qr/^ /;

# skip a new-line beginning with a non-printing character
# \s = space, \t = tab, \r = carriage-return, \n = new-line

my $rgx2 = qr/^\s/;


##</code><code>##


# $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


##</code><code>##


my $regex = $n == 1 ? $rgx1 : $n==2 ? $rgx2 : $rgx3 ;

unless( $n == 3 ){

   while(<INPUT>){

     next if $regex;   

     tr* *,*  ; # replace all spaces with a comma
     tr* *,*s ; # replace all squeezed spaces with a comma

     next;
   }

}else{

   die 'regex number choice fail' unless $n == 3;
 
   #slurp in input for multi-line parse
   my $input = do { local $/ = undef; <INPUT> };

   # apply substitution operator with regex to string;


   $input =~ 
   s/$rgx3/ $1 =~ /^\s/ ? $2 =~ tr* *,* : $2 /gme;  

   # each sentence substituted, if starts with extra
   # leading spaces, substitutes commas.
   # else substitues existing sentence.
   # untested. thx to Cristoforo for idea of tr in e
}