After much pain I find that the inner "for loop" preserves the outer $_ but that the inner "while loop" clobbers the outer $_. What is the general principle for the clobbering-nonclobbering of the inner $_ (you need to type something and hit return to finish the while loop). 
use strict; use diagnostics;
my @aa = ([1,2],[3,4]);
for (@aa) {
    for (1..3) {
        print $_;
        last;
    } 
print "forfor=$_\n";
}
my @bb = ([5,6],[7,8]);
for (@bb) {
    while (<STDIN>) {
        print "while=$_\n";
        last;
    } 
print "forwhile=$_\n";
}

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In reply to While but not for clobbers $_ by tenya

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