Something like:

use strict;
use warnings;
use Data::Dumper;

my %hash;
while (my $line = <DATA> ) {
    chomp $line;
    my ( $key, $value ) = split /\s*::=\s*/, $line;
    if ( $key and $value ) {
        push @{$hash{$key}}, $value;
    }
}


print Dumper \%hash;

__DATA__
A::=B
C::=D
A::=C
[download]

This assumes one entry per line. If there can be more than that, you'll need to explain what delimits each entry. The conditional (if) is just a very simplistic approach toward rejecting blank lines or mismatched key/value pairs. I also added the flexibility of allowing whitespace between the key, the ::=, and the value. As you can see when you run this snippet, my solution creates what you asked for, a hash where each element is an array of values for the appropriate key.