This probably has all the efficiency of Schlemiel the Painter, but it has some humor value:

!$array[$_] && $string =~ s/(?<=^.{$_})(.)/lc $1/e for 0 .. $#array;
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Silly at best, but it works. :)

This isn't as fun, but doesn't suffer from a lookbehind assertion that grows as the index increases:

!$array[$_] and substr( $string, $_, 1 ) = lc substr( $string, $_, 1 )
    for 0 .. $#array;
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