Would anonymous subroutines do what you need?

my $a = sub { return $_[0] + $_[1]; };
print $a->(3, 5);
[download]

Edit: if you really need to do it runtime -- that is, dynamically, how about this:

my $code = 'return $_[0] + $_[1];'
my $a = eval "sub { $code }";
print $a->(3, 5);
[download]

One thing to consider is that $a is a special variable, used by sort, so you might not want to use that name, but the technique still holds I think.