# use like this:
%pets = hash_slice( \%hash, qw( dog cat ) );
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For once, this may be a situation in which using a prototype would not be that bad:

sub hash_slice (\%@) {
    my $h=shift;
    map { $_ => $h->{$_} } @_;
}
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(Or the other way with slices. I don't really care.)