What version of Perl are you using? When I run your code on my system (WinXP, ActivePerl 5.8.8), I get an incorrect answer. I added a the use warnings pragma to your code and got this result:

C:\> test.pl
Argument "0x2" isn't numeric in addition (+) at C:\test.pl line 9.
Argument "0x28" isn't numeric in addition (+) at C:\test.pl line 9.
0 0
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You can specify hexadecimals in literal code by appending an '0x' to the value. I suppose, if you really wanted to get to a hexadecimal from a string, you could use string eval.

my $this = sprintf("0x%x", 40);
my $that = sprintf("0x%x", 2);

my $other = eval "$this + $that";

printf "$other %x\n", $other;
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I'd be inclined to go with hex, though.