use Regexp::List qw( );

my @known = qw(
   eacute
   Eacute
   ecirc
   Ecirc
   ...
);

my $known = Regexp::List->new()->list2re(@known);

s/
   &
   (?!
      (?:
         \# (?: x[\da-f]+ | \d+ )
      |
         $known
      )
      ;
   )
/&/xi