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