use strict;
use warnings;

my $string =
   qq{one two   \n\n   three\t\t\n four    five\n\n\n\t six \n\n};

my $newString =
   join  qq{\n},
   map   {
             s{^\s+}{};
             s{\s+$}{};
             s{\s+}{ }g;
             $_;
         }
   split m{\n+}, $string;

print qq{->$newString<-\n};