grep: Picking Items from a List

perl -le 'my @odd_numbers = grep { $_ % 2 } 1..1000; print @odd_numbers;'

While the grep is running, $_ is aliased to one element of the list after another. You’ve
seen this behavior before, in the foreach loop. It’s generally a bad idea to modify $_
inside the grep expression because this will damage the original data.

perl -le '@files = grep{/*.pl/i} glob "*.*"; print @files;'
my @matching_lines = grep { /\bfred\b/i } ; the same with
my @matching_lines = grep /\bfred\b/i, ;

#!/usr/bin/perl -w
#
use strict;

my @input_numbers = (1, 2, 4, 8, 16, 32, 64);
my @my_num1 = grep $_ > 10, @input_numbers;
my @num1 = grep {!($_ % 4)} @input_numbers;
my @num2 = grep /4$/, @input_numbers;
print "@my_num1, @num2, @num1\n";

my @odd_digit_sum = grep digit_sum_is_odd($_), @input_numbers;
print "@odd_digit_sum\n";#output is 1 16 32

sub digit_sum_is_odd {
    my $input = shift;
    my @digits = split //, $input;  # Assume no nondigit characters
    my $sum;
    $sum += $_ for @digits;
    return $sum % 2 ;
}

we can also use below code to complete.

my @r = grep { my $num = $_;
               my $re;
               $re += $_ for split//, $num;
               $re %2;
}(10,30,20,13);#out put 10,30

my %provisions = (
  'The Skipper'   => [qw(blue_shirt hat jacket preserver sunscreen)],
  'The Professor' => [qw(sunscreen water_bottle slide_rule radio)  ],
  'Gilligan'      => [qw(red_shirt hat lucky_socks water_bottle)   ],
);

my @need = grep { my @k = @{$provisions{$_}};
                  grep {$_ eq 'water_bottle' } @k;
}keys %provisions;
print "@need";  #out put Gilligan The Professor