•         elsif ($g == 255){
              $idx =  @master_ascii;
          }
  [download]

  should be

          elsif ($g == 255){
              $idx =  $#master_ascii;
          }
  [download]

  which allows us to change

  my $char = $master_ascii[$idx];
  print defined $char ? $char : ' ';
  [download]

  to

  print $master_ascii[$idx];
  [download]

• my @master_ascii = (qw(W M 0 @ N & ), ',', qw(' - . `), ' ');
  [download]

  can be simplified to

  my @master_ascii = (qw(W M 0 @ N & , ' - . `), ' ');
  [download]

• Also,

  $idx = int((scalar @master_ascii) * $g / 256);
  [download]

  can be simplified to

  $idx = @master_ascii * $g / 256;
  [download]

  (Multiplication already imposes a scalar context, and there's an implied int around values used as array indexes.)

Update: It would be extra cool is you compressed the image vertically to preserve the aspect ratio.