When I read the subject ("while nested in foreach"), I thought this would be a different problem. Just for grins, here is a "gotcha" to watch for: resetting $_.

for (1..10){
    print "Before nested \$_ setter: $_\n";

    open JUNK, 'junk' or die "Nope: $!";
    while(<JUNK>){
        # Do nothing, just let $_ get set
    }
    close JUNK;

    print "After  nested \$_ setter: $_\n";
}
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In this example (perl 5.8.0), the second print statement shows an empty $_ because the diamond operator (in the while loop) overwrote $_ (as we told it to do). So, after the nested while loop, we no longer have the old value in $_ (the counter in the for loop).

Exercises left for the reader: replace the while loop with other $_-using constructs and see how they behave...

Most perl built-ins, and many CPAN modules, are "smart" enough to localize $_ to prevent confusion, but not all.