my ($from, $to, @to);
foreach (<DATA>) {
    chomp($_);
    ($from, $to) = split(/ -> /, $_);
    @to = split(/ /, $to);
    foreach (@to) {
        $hash{$_}{$from} = ();
    }
}
foreach (sort keys %hash) {
    print "$_ <-";
    $to = $hash{$_};
    foreach (sort keys %$to) {
        print " $_";
    }
    print "\n";
}

__DATA__
page1.html -> page2.html page3.html
page2.html -> page3.html page4.html
page3.html -> page4.html
page4.html -> page1.html