my $text = 'hello
   this is a multiline
   message';

$text =~ s/([^\S\n]+|\n)/$1 eq "\n" ?
                         print "New line\n" :
                         print "Whitespaces\n"; $1/gemi;

##</code><code>##

New line
Whitespaces
Whitespaces
Whitespaces
Whitespaces
New line
Whitespaces