You could use CGI::Ex::Template which has exposed the expression parsing methods from the Template::Toolkit mini-language. This gives you all of the access to numeric extensions. You can also get support for variables (though you have to use the Template::Toolkit expression syntax for setting a variable (encasing the set expression in parens). And you can use the TT virtual methods (plus filters exposed by CGI::Ex::Template).

use strict;
use CGI::Ex::Template;
my $t = CGI::Ex::Template->new;

### hmm - semi documented
$t->_vars({
  PI => 3.14159,
  r  => 1.5,
});

foreach ("2 + 3",
         "1 + (2 + 4) / 3",
         "2 ** 3 ** 4",
         '12345.fmt("%.3e")',
         '0xFF',
         "PI",
         "PI * r ** 2",
         "1 + (",
         "foo",
         "(foo = 123456 + 1)",
         "foo",
         ) {
    my $expr = $_;
    print "-------------\n";
    print "Expression: $expr\n";
    my $optree = eval { $t->parse_expr(\$expr) };
    #use CGI::Ex::Dump qw(debug);
    #debug $optree;

    my $result;
    if (my $err = $@) {
        print "Error:      ".(ref $err ? $err->info : "$err")."\n";
    } else {
        $result = $t->play_expr($optree);
        print "Result:     $result\n";
    }

}
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That chunk of code would print out the following:

-------------
Expression: 2 + 3
Result:     5
-------------
Expression: 1 + (2 + 4) / 3
Result:     3
-------------
Expression: 2 ** 3 ** 4
Result:     2.41785163922926e+24
-------------
Expression: 12345.fmt("%.3e")
Result:     1.234e+04
-------------
Expression: 0xFF
Result:     255
-------------
Expression: PI
Result:     3.14159
-------------
Expression: PI * r ** 2
Result:     7.0685775
-------------
Expression: 1 + (
Error:      Missing close )
-------------
Expression: foo
Result:
-------------
Expression: (foo = 123456 + 1)
Result:     123457
-------------
Expression: foo
Result:     123457
[download]

I also noticed an interesting thing, the parsing of CGI:Ex::Template would turn 1 + * 3 into 1 + undef * 3 which would evaluate to 1 when you may have wanted it to report an error. I'll see if we can get a fix in for CGI::Ex::Template.