my @set1 = ...;

my %h1 = map { $_ => 0 } @set1;
my %h2;
( $h1{ $_ } // $h2{ $_ } )++ for @list;

##</code><code>##

my @set1 = ...;
my %set1 = map { $_ => 1 } @set1;

my ( %h1, %h2 );
( $set1{ $_ } ? $h1{ $_ } : $h2{ $_ } )++ for @list;