How is this structure called?

That's an (anonymous, no named variable) array reference which holds hash references. See perlref, and really! Do read it.

use JSON;
use Data::Dump "dump";

dump( other_way() ); # This will show the anonymous structure.

print "JSON: ", to_json( other_way() ), $/;

sub other_way {
    my %user1 = ( username => "paco",
                  fish => "Sunfish" );
    my %user2 = ( username => "YourUncle",
                  fish => "Coelacanth" );
    my %user3 = ( username => "hiragana",
                  fish => "Monkfish" );
    my %user4 = ( username => "MosaicNL",
                  fish => "Sixgill Shark" );
    my @users = ( \%user1, \%user2, \%user3, \%user4 );

    return \@users;
}
[download]

NB: the dump() can appear "out of order" because it goes to STDERR. Output-

[
  { fish => "Sunfish", username => "paco" },
  { fish => "Coelacanth", username => "YourUncle" },
  { fish => "Monkfish", username => "hiragana" },
  { fish => "Sixgill Shark", username => "MosaicNL" },
]
JSON: [{"fish":"Sunfish","username":"paco"},{"fish":"Coelacanth","user
+name":"YourUncle"},{"fish":"Monkfish","username":"hiragana"},{"fish":
+"Sixgill Shark","username":"MosaicNL"}]
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You can see in &other_way above how much more terse and more clean anonymous structures tend to be.