how do I determine if any of those links match the leading one I used to start the fetch, and subsequent links found on pages followed from there?

Strip any anchors off immediately upon capturing the link. Keep a single hash with all of the seen URLs. When you start, add your root URL to the hash.

If I blindly just push the new elements onto the hash, I'll overwrite any existing keys (and values) which have the same key, which might not be good for efficiency.

You won't overwrite the keys. Perl will see that the key exists and simply change the value. You can use exists() if you want to, but it isn't really necessary and I wouldn't do it.

-sauoq
"My two cents aren't worth a dime.";

If I'm guessing right about what you are trying to do, here's some code that might help. You'll have to provide the fetch_urls() sub and you should also think about limiting the depth and/or breadth of your search.

Untested...

my %seen;
my @urls; 
my $root_url = 'http://url.example.com/'; 

$seen{$root_url}++;
push @urls, $root_url; 

while ($root_url = shift @urls) {  
    for my $url ( fetch_urls($root_url) ) { 
        $url =~ s/#.*$//;

        push @urls, $url unless $seen{$url};
        $seen{$url}++;
    }
}
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-sauoq
"My two cents aren't worth a dime.";