Dynamically generated GIF is a broken image in OSX High Sierra and Safari

I have a Perl function that dynamically generates a captcha as a GIF image. This is an actual GIF file generated by this function...

In the latest Safari (and Safari Technology Preview) v12 in High Sierra (Mac OS X version 10.13.x), this same GIF appears as a broken image icon:

It also will not display in High Sierra's version of the Preview application or within the "preview" pane of the "Get Info" window.

This Perl has been in place for many years and, until now, working on all browsers. Firefox and Chrome in High Sierra have no problem properly displaying this GIF.

Perl:

sub image {
  my ($key) = @_;
  $key =~ s/\D//g;
  my $buff;
  my $bpx = 7;
  my $pal = "\x33\x33\x33" .
            "\xff\xff\xff" .
            "\x66\x66\x66" .
            "\x00\x00\x00";

  initcmx();

  my $dh = int(length($dot) / $dw);
  my $ch = int(length($cmx{0}) / $cw);
  my $len = length($key);
  my $w = $len * $cw * $dw;
  my $h = $ch * $dh;
  my $blmt = 2**$bpx - 3;

# GIF header
  $buff .= 'GIF87a';
  $buff .= pack 'v2', $w, $h;
  $buff .= pack 'C', 0xF0 + $bpx - 1;
  $buff .= "\0";
  $buff .= "\0";
  $buff .= $pal;
  $buff .= "\0" x ((2**$bpx * 3) - length $pal);
  $buff .= ',';
  $buff .= "\0\0\0\0";
  $buff .= pack 'v2', $w, $h;
  $buff .= "\0";
  $buff .= pack 'C', $bpx;
#
# Build image
#
  my $img;
  for (my $y = 0; $y < $h; $y++) {
    my $cy = int($y / $dh) % $ch;
    my $dy = $y % $dh;
    for (my $x = 0; $x < $w; $x += $dw) {
      my $cx = int($x / $dw) % $cw;
      my $i = int($x / $dw / $cw);
      my $c = substr $key, $i, 1;
      my $d = substr $cmx{$c}, $cy * $cw + $cx, 1;
      my $di = ($d eq 'X') ? $dot : $nodot;
      $di = substr $di, $dy * $dw, $dw;
      for ($i = 0; $i < length $di; $i++) {
        $c = ord substr $di, $i, 1;
        $c = chr $c;
        $img .= $c;
      }
    }
  }
#
# Pack image
#
  my $cnt;
  for (my $i = 0; $i < length $img; $i += $cnt) {
    $cnt = (length $img) - $i;
    if ($cnt > $blmt) {
      $cnt = $blmt;
    }
    $buff .= pack 'C', $cnt + 1;
    $buff .= substr $img, $i, $cnt;
    $buff .= pack 'C', 2**$bpx;
  }
  $buff .= pack 'C', 2**$bpx + 1;
  $buff .= "\0";
  $buff .= ';';
  return "Content-type: image/gif\nExpires: Wed, 05-Jan-2007 01:01:01 GMT\n\n".$buff;
}
# ====================================================================== initcmx
#
# Quick and dirty solution. The matrix must be loadable
#

sub initcmx {

  $cw = 6;

$cmx{'0'} = qq~
......
.XXX..
X...X.
X...X.
X...X.
X...X.
X...X.
.XXX..
......
~;
$cmx{'1'} = qq~
..X...
.XX...
..X...
..X...
..X...
..X...
.XXX..
......
......
~;
if (rand() < 0.5) {
$cmx{'1'} = qq~
......
...X..
..XX..
...X..
...X..
...X..
...X..
...X..
..XXX.
~;
}
$cmx{'2'} = qq~
......
......
.XXX..
X...X.
....X.
..XX..
.X....
X.....
XXXXX.
~;
if (rand() < 0.3) {
$cmx{'2'} = qq~
.XXX..
X...X.
....X.
..XX..
.X....
X.....
XXXXX.
......
......
~;
}
if (rand() > 0.7) {
$cmx{'2'} = qq~
......
XXXX..
....X.
....X.
.XXX..
X.....
X.....
XXXXX.
......
~;
}
$cmx{'3'} = qq~
XXXXX.
....X.
...X..
..XX..
....X.
X...X.
.XXX..
......
......
~;
$cmx{'4'} = qq~
......
...X..
..XX..
.X.X..
X..X..
XXXXX.
...X..
...X..
......
~;
if (rand() < 0.5) {
$cmx{'4'} = qq~
....X.
...XX.
..X.X.
.X..X.
XXXXX.
....X.
....X.
......
......
~;
}
$cmx{'5'} = qq~
......
......
XXXXX.
X.....
XXXX..
....X.
....X.
X...X.
.XXX..
~;
$cmx{'6'} = qq~
..XXX.
.X....
X.....
XXXX..
X...X.
X...X.
.XXX..
......
......
~;
if (rand() < 0.5) {
$cmx{'6'} = qq~
......
......
..XXX.
.X....
X.....
XXXX..
X...X.
X...X.
.XXX..
~;
}
$cmx{'7'} = qq~
......
XXXXX.
....X.
...X..
..X...
.X....
.X....
.X....
......
~;
if (rand() < 0.5) {
$cmx{'7'} = qq~
......
......
XXXXXX
.....X
....X.
...X..
..X...
.X....
X.....
~;
}
$cmx{'8'} = qq~
.XXX..
X...X.
X...X.
.XXX..
X...X.
X...X.
.XXX..
......
......
~;
$cmx{'9'} = qq~
......
......
.XXX..
X...X.
X...X.
.XXXX.
....X.
...X..
XXX...
~;
  for (my $i = 0; $i < 10; ++$i) {
    $cmx{$i} =~ s/\n//g;
    $cmx{$i} =~ s/\r//g;
  }
  $dot = qq~
\1\2\1
\2\3\2
\1\2\1
~;
  $nodot = qq~
\0\0\0
\0\0\0
\0\0\0
~;
 $dot =~ s/\n//g;
 $dot =~ s/\r//g;
 $nodot =~ s/\n//g;
 $nodot =~ s/\r//g;
 $dw = 3;
}

Does High Sierra have a problem with GIF87a in general or is there something wrong with the way this particular file is being generated? Google searching has been fruitless as most people are only reporting various issues with animated GIFs.

EDIT: I've since rewrote my Perl to create PNG instead of GIF files. However, I'm leaving this question in place as I'm still curious about what causes this issue.