for (;;) {
   print("Enter Number: ");
   defined( my $line = <STDIN> )
      or last;
   s/^\s+//, s/\s+\z// for $line;
   next if $line eq '';
   last if $line =~ /^(?:d|done)\z/i;
   ...
}

##</code><code>##

while ( (print "Enter Number: "), 
        (my $line=<STDIN>) !~ /^\s*d(?:one)?\s*$/i  
      )
{
   next if $line =~ /^\s*$/;  #re-prompt on blank lines
   ...
      chomp($line);
   ...
}