I just did a test on $_ using:

foreach ( qw(a b c) )
{
    print $_."\n";
}
[download]

1. It shows up under "Global Variables" not perl internal variables
2. It only shows up when $_ is in scope, so it disappears once I exit the loop, and if I rewrite the for loop above with a named temporary variable, then it never appears.

I was testing under linux, but with the same version of Eclipse and perl, so it should be comparable to your setup.