I do not know what you used to learn Perl, but clearly it's a bad resource/teacher if you already confused by this very basic problem. See http://learn.perl.org/ and http://perl-tutorial.org/ to find good ones.

You need to declare variables you use with the my function.

use strict;
use warnings FATAL => 'all';
my %dns_check = (address => 'ip');

while (my ($key, $value) = each %dns_check) {
    my $found_addr = gethostbyname($key) or die "Error $!\n";
    $found_addr = inet_ntoa(inet_aton($key));
    print "IP found is: $found_addr\n";
}
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