You could combine a split, a map containing regex substitutions, and a join.

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};
[download]

The output.

->one two
three
four five
six<-
[download]

I hope this is useful.

Cheers,

JohnGG

Update: Just realised that the third substitution in the map should be global. Corrected.