Try to read perlintro or perldata for more information on hashes, and perlreftut for more involved data structures. Suppose you have a comment stored in $comment, and want to store that it's related to the url $post_url, you'd write:

my %comments;
...
push @{$comments{$post_url}}, $comment;
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And you can retrieve and iterate over the list of comments to an URL:

for my $c (@{$comments{$post_url}}) {
    print "$c\n";
}
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