I will do it this way:

use strict;
use warnings;

my $e = "foo(0)";
eval $e;

sub foo {
    my $i = shift;
    print "$i foo\n";
    if ($i < 10){
        $i++;
        print "calling foo\n";
        foo($i);    
    }
}
[download]

Other than the direct issue others already pointed out, there is two other style things:

1. $i is not well scoped. You never define a variable with a scope even slightly bigger than its need. In case, $i shall be defined within the foo() function, and be passed as a parameter when it calls itself.
2. It might seems okay for a demo, but define a sub in a string is not quite a good idea.

Your guess that it has something to do with multithreading is... as multithreading will not help in this case.