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