Something like this ought to work, though if it were me I would prefer providing the user with some context if I'm going to prompt them to remove an <alt> tag.

use IO::Prompt::Tiny q(prompt);

sub remove_ok {
    my $prompt = 'Do you want to remove <alt> tags? (y/n)';
    my $default = 'n';

    my $resp = '';
    while ($resp !~ m/^[yn]/i) {
        $resp = prompt($prompt, $default);
    }
    return $resp =~ m/^y/i;
}

sub RemoveAltTag {
    my $doc = shift;

    my $cnt = 0;
    my $nodes = $doc->getElementsByTagName("image");

    for(my $i = 0;$i < $nodes->getLength(); $i++) {
        my $kids = $nodes->item($i)->getChildNodes();

        for(my $k =0; $k < $kids->getLength(); $k++) {

            if(
                    $kids->item($k)->toString() =~ /<alt>/i
                &&  remove_ok()
            ) {
                $nodes->item($i)->removeChild($kids->item($k));

                print "\n Removed <alt> tag" if $VERBOSE;

                $cnt++
            }
        }
    }

    return $cnt;
}
[download]

But again, you might want to show some of the surrounding context so that the user can use that information in deciding what to remove.