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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In reply to Re: Changing $/ mid-file by eg
in thread Changing $/ mid-file by jgallagher

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