On further investigation, it appears that you can tell the two contexts apart using the Want module. Specifically, want('BOOL') knows the difference. For example: 
my $flipflop = 0;

sub foo {
    use Want qw(want);

    print 'not ' if !want 'BOOL';
    print "bool\n";

    return $flipflop=!$flipflop;   # alternate true and false
}

for (1..3) {
    while (foo() ) {
        # print "here 1\n";
    }
    while ($answer = foo() ) {
        # print "here 2\n";
    }
}

[download]

Unfortunately, under 5.8.3 I get bus errors if there I uncomment the print statements, or if there is anything in the while blocks. :-(

In reply to Re^4: sub that sets $_ by TheDamian
in thread sub that sets $_ by holli

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