The general structure of your code could look like (untested)

my $string = ''; # output string
open FILE, "<file.txt"
   or die "Could not open file.txt: $!";
while (<FILE>) {
   chomp;
   s/^N\d+ //; # strip line numbers
   if (/^\*/) { # comment
      s/^\*/!/;
      $string .= $_;
   }
   else { # other
      while ($_ =~ /[x-z]/([\d.+-]+)/ig) {
             $string .= " $1";
         }
   }
}
print $string;
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-Mark