use HTML::TokeParser;
use LWP::Simple;

my $content = get('http://www.yoursite.com');

my (@alt, $link);

my $parser = HTML::TokeParser->new(\$content) || die $!;
while (my $token = $parser->get_token) {
    my $type = shift @{$token};
    if ($type eq 'E') {
        my ($tag) = @{$token};
        $link = 0 if $tag eq 'a';
    } elsif ($type eq 'S') {
        my ($tag, $attr, $attrseq, $text) = @{$token};
        $link = 1 if $tag eq 'a';
        next unless $tag eq 'img';
        next unless defined $attr->{'alt'} and length $attr->{'alt'};
        push @alt, { $attr->{'src'} => $attr->{'alt'} } if $link;
    }
}