Ummm, because your $test is quoted inside double quote, your $test = "D and A" even before you go into the eval, so the eval had no effect.

Change the code slightly as follows -

my $alpha = "A";
my $delta = "D";
my $test = '$delta and $alpha';
eval "\$test = \"$test\"";
print "$test";
[download]

and yes, it will work this time, because the code is equivalent to -

my $alpha = "A";
my $delta = "D";
my $test = '$delta and $alpha';
$test = "$delta and $alpha";
print "$test";
[download]

Where the variable $test inside double quote " " has been expanded by the perl interpreter to the value of $test before going into eval