++$Y{$y} if grep { $y eq $_ } @ARGV;

##</code><code>##

my $re;
if ( @ARGV ) {
   my $pat = join "|", map quotemeta, @ARGV;
   $re = qr/^(?:$pat)\z/;
}

my %Y;
while ( <> ) {
  my ( $y ) = ...;

  ++$Y{ $y } if $y && ( !$re || $y =~ $re );
}

##</code><code>##

my $re;
my %valid;
++$valid{ $_ } for @ARGV;

my %Y;
while ( <> ) {
  my ( $y ) = ...;

  ++$Y{ $y } if $y && ( !%valid || $valid{ $_ } );
}