And as always there's a module on CPAN to do this very thing

use Math::Expression;

my $me = Math::Expression->new;
print $me->Eval( $me->Parse("2 + 2") );

__output__

4
[download]

Not as slimline as string eval, but not as potentially dangerous either :) See. the Math::Expression docs for more info.
HTH

_________
broquaint