You are confusing the things a little bit. When you do @x = @{$a} you are "copying" the content of "@{$a}" to "@x". With foreach, you are just passing a reference to it...
If you don' wanna mess with your original array, you need to copy it to a second array (like you did @x=@{$a}) and use @x ...

@x = @{$a};
foreach my $i (@x) {
        print"$i\n";
        undef $i;
}
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or as someone suggested before:

my $a = ['a', 'b', 'c'];
foreach  (@{$a}) {
        my $i = $_;
        print"$i\n";
        undef $i;
}
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