First of all, the syntax in the sub is incorrect:

%s{'a'} = 10 ;
%s{'b'} = 20 ;
%s{'c'} = 30 ;
[download]

should be

$s{'a'} = 10 ;
$s{'b'} = 20 ;
$s{'c'} = 30 ;
[download]

anyway, the following change will work for you:

#!/usr/bin/perl
use strict  ;
use warnings ;


foreach ( keys %{&testa} ) {
    print "key is $_\n" ;
}

sub testa {
    my $s = () ;
    $s->{'a'} = 10 ;
    $s->{'b'} = 20 ;
    $s->{'c'} = 30 ;

    return $s ;
}
[download]

hope this helps,
davidj