You could use an anonymous subroutine as the hash value for 'text':

%hash = (
    "text" => sub {
        my $color_opt = shift;
        return <<"TEXTEND",
We play the same game again, but with a different proportion of colore
+d balls in
+ Box K (see below).  Everything else is the same. 
<p>
You can choose between box U or box K, both containing 100 balls 
of five different colors. One ball will be drawn from the box you have
+ chosen. 
You win &#36;1,000 if a $color_opt ball is drawn.
<p>
Please select the box of your choice: U or K. 
If you think both boxes are equally attractive, you can select Indiffe
+rent.
TEXTEND
    },
);
print $hash{text}->("blue");
print $hash{text}->("chartreuse");
[download]

That way the interpolation is done when you evaluate the hash value (i.e. call the subroutine).