As LanX said, it's just a parenthesis-less method call to "get". "=>" is often called a "fat comma" in Perl; but more over it's like passing an anonymous hash to the "get" method as implemented by Mojo.
IOW, it's also equivalent to the following where "/" is the hash key and anonymous subroutine is the hash value:

my $hash_ref = { '/' => sub {...} };
get($hash_ref);
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And for that matter,

my $array_ref = [ '/', sub {...} ];
get($array_ref);
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Ultimately, you are associating the subroutine handler for the URL; in this case, "/".