$hash{$_} ||= 1 + keys %hash for @list;
$hash{$_} = $hash{$_} || 1 + keys %hash for @list;

##</code><code>##

for ( @list ) {
    $hash{$_} ||= ( 1 + keys %hash );
}

for ( @list ) {
    $hash{$_} = ( $hash{$_} || ( 1 + keys %hash ) );
}

##</code><code>##

$hash{$_} ||= 1 + keys %hash;

# looks more like:

( $hash{$_} = $hash{$_} ) || ( $hash{$_} = 1 + keys %hash )