If you take out the second while, it looks fine to me. Try this:

while (<DATA>) {
        chomp; 
        print "-$_-\n";
        if (/aaaa$/) { 
                $/ = '%'; 
        } 
}
__END__
XXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXX
XXXXXXXXXXXXaaaa
abcdefghijklm%nopqrstuvw%xyz
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