extract_interesting_keys.pl

#!/usr/bin/perl

use Data::Dumper;

my @interesting_keys = qw/a/;

my $hash_list = [ { a => 1, b => 1 }, { a => 2 } ];
my $new_hash_list = [
    map {
        my $dest = $_;
        +{
           map { $_ => $dest->{$_} } @interesting_keys
        };
    } @{$hash_list}
];
print Dumper($new_hash_list);

my $new_hash_list2 = [];
for my $dest (@{$hash_list}) {
    my $new_dest;
    @{$new_dest}{@interesting_keys} = @{$dest}{@interesting_keys};
    push @{$new_hash_list2}, $new_dest;
}
print Dumper($new_hash_list2);

# in place editing!
my %is_interesting = map { $_ => 1 } @interesting_keys;
for my $dest (@{$hash_list}) {
    for my $key (keys %{$dest}) {
        delete $dest->{$key} unless $is_interesting{$key}
    }
}
print Dumper($hash_list);

__END__

$VAR1 = [
          {
            'a' => 1
          },
          {
            'a' => 2
          }
        ];
$VAR1 = [
          {
            'a' => 1
          },
          {
            'a' => 2
          }
        ];
$VAR1 = [
          {
            'a' => 1
          },
          {
            'a' => 2
          }
        ];