The reason is history. The ability to write:

    foreach my $variable (LIST) {...}
[download]

was relatively recent (5.004 I think). Before one had to write

    my $variable;
    foreach $variable (LIST) {...}
[download]

As usual, it's combining old Perl DWIM, new features and backwards compatability that leads to oddities like this. Luckely, for most people, it all just works, and old programs don't break.