What about running the tests, then modifying the sub, then running again? Here's a quickly thrown together example (in this example I simply mock out the whole function) to see if this is more along the lines of what you're looking for:

package Package; {

    sub perform {
        return $_[0] + $_[1];
    }
}

package main; {
    use Mock::Sub;
    use Test::More;

    tests();

    my $m = Mock::Sub->new;
    my $changed = $m->mock('Package::perform');
    $changed->return_value($_[0] - $_[1]);

    tests();

    done_testing();

    sub tests {
        is Package::perform(5, 5), 10, "5 + 5 = 10";
    } 
}
[download]

Output:

ok 1 - 5 + 5 = 10
not ok 2 - 5 + 5 = 10
#   Failed test '5 + 5 = 10'
#   at pack.pl line 24.
#          got: '0'
#     expected: '10'
1..2
# Looks like you failed 1 test of 2.
[download]

If this example is more along the lines you're after (modifying code on the fly), my Devel::Examine::Subs is designed to alter code within a file (so say you wanted to modify a single line in a single sub within a package, you could (then revert it back), and I could write a mock up example of what it may look like. But perhaps I'm way off here.