while( <$infh> ) {
        s{&}{&amp;}g; ## In some case does not match as intended
        s{&amp;amp;}{&amp;}g;
        ...
}

##</code><code>##

14:25 >perl -wE "my @s = ('Fred & Wilma', 'Barney &amp; Betty'); for (@s) { s{&(?!amp;)}{&amp;}g }; say for @s;"
Fred &amp; Wilma
Barney &amp; Betty

14:25 >