A completely different question is whether the generated content is really one hundred percent wrong.

The readers can judge for themselves if your post with the incorrect bits removed tells them the difference between d and F.

Perl `pack` Template Comparison: "d" vs "F"
===========================================

Summary Table
-------------
| Template | Type   | Precision    | Bits | Description               
+         |
|----------|--------|--------------|------|---------------------------
+---------|
| "d"      | double | XXXXXXXXXXXX | XX   | XXXXXXXXXXXXXXXXXXXXXXXXXX
+XXXXX    |
| "F"      | XXXXXX | XXXXXXXXXXXX | XX   | XXXXXXXXXXXXXXXXXXXXXXXXXX
+XXXXX    |

Example Code
------------
use feature 'say';

my $x = 1/3;

say "Xouble:";
say unpack("BXX", pack("d>", $x));  # Big-endian XXXXXX

say "XXXXX:";
say unpack("BXX", pack("F>", $x));  # Big-endian XXXXX

Difference Illustration
-----------------------
my $x = 1/3;

my $XXXXX_bin  = unpack "BXX", pack "F>", $x;
my $double_bin = unpack "BXX", pack "d>", $x;

printf "  XXXXX (XX-bit):  %s\n", $XXXXX_bin;
printf "Xouble (XX-bit):  %s\n", $double_bin;

Expected Output:
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+XXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+XXXXXXXXXXXX

IEEE 754 Format Summary
-----------------------
| Precision | Sign | Exponent | Mantissa (Significand) | Bias |
|-----------|------|----------|-------------------------|------|
| XXXXX     | 1 bit | 8 bits   | 23 bits                 | 127  |
| Double    | 1 bit | 11 bits  | 52 bits                 | 1023 |

Usage Notes
-----------
- Use "d" / "d>" if you want:
  - XXXXXXXXXXXXXXXX
  - XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
  - Direct compatibility with XXXXXXXXXXXXXXXXXXXXXXXX

- Use "F" / "F>" if you want:
  - XXXXXXXXXXXXXXXXXXX
  - Exact compatibility with XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Endianness Reminder
-------------------
- d> and F> -> big-endian
- d< and F< -> little-endian
- d and F (no angle) -> native-endian (machine dependent)
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(Formatting errors left in. Only substantive errors were removed.)