Nice work, LanX, thank you!
I tried that in a script, getting it to read all the lines of data, and it works perfectly!
Here it is:

#!/usr/bin/perl

while (<DATA>)
{
        #($pre, $ranges) = split /=/, $_, 2;
        ($pre, $ranges, $value) = $_ =~ /^(.+?)=([^ ]+) (.+)$/;

        $del = "=";
        @keys = $pre;
        for $mult (split /:/, $ranges)
        {
                @keys = map { $a=$_; map { "$a$del$_" } split /,/, $mu
+lt } @keys;
                $del = ':';
        }
        #@hash{@keys} = ('value2') x @keys;
        @hash{@keys} = ($value) x @keys;
}

use Data::Dump 'dd'; dd \%hash;

__DATA__
Prefix1=A,B:c,d value1
Prefix2=A:b,c:1,2 value2
Prefix3=A:*:1,2 value3
[download]

And the output:

{
  "Prefix1=A:c"   => "value1",
  "Prefix1=A:d"   => "value1",
  "Prefix1=B:c"   => "value1",
  "Prefix1=B:d"   => "value1",
  "Prefix2=A:b:1" => "value2",
  "Prefix2=A:b:2" => "value2",
  "Prefix2=A:c:1" => "value2",
  "Prefix2=A:c:2" => "value2",
  "Prefix3=A:*:1" => "value3",
  "Prefix3=A:*:2" => "value3",
}
[download]

If you have any suggested changes to my changes, I'm all ears.
I've also added 4 updates to my original post in case you're interested.  No worries if not. You can ignore update #1.