If you were using warnings you should/would be getting these sorts of warnings:

Use of uninitialized value in delete at
Attempt to free unreferenced scalar
[download]

What's funny is that when I run this bad code:

%hsh = qw(a 1 b 2 c 3 d 4);

for $key (%hsh) {
 delete $hsh{$key};
}
$num = %hsh;
print "$num\n";
[download]

I get alot of warnings then a core dump. If I add 'use warnings' (or -w or $^W = 1), I get a few warnings and I don't get the core dump. Are you using strict and warnings? My guess is no, since if you were that line would be:

foreach my $session (%Sessions) ...