Hello Ppeoc,

If efficiency is really important, you should use a short-circuiting function rather than grep, which evaluates all instances in the input list. For example, from List::Util#any:

Many cases of using grep in a conditional can be written using any instead, as it can short-circuit after the first true result.

BTW, are you certain that @Single and @Double will never have values in common? If not, you may need to change the logic to incorporate a fourth flag:

use strict;
use warnings;
use Data::Dump;
use List::Util qw( any );

my @choices = qw/Apple Banana Tiger Lion Cat Turtle/;
my @Single  = qw/Apple Turtle/;
my @Double  = qw/Apple Lion/;
my %choices;

for my $field (@choices)
{
    my $flag = 0;

    $flag  = 1 if any { $_ eq $field } @Single;
    $flag += 2 if any { $_ eq $field } @Double;

    $choices{$field} = $flag;
}

# $flag == 3 means $field is present in both @Single and @Double

dd \%choices;
[download]

Output:

14:27 >perl 1508_SoPW.pl
{ Apple => 3, Banana => 0, Cat => 0, Lion => 2, Tiger => 0, Turtle => 
+1 }

14:28 >
[download]

Hope that helps,