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ShoppingCart->new(
    user => $user,
    map { $_->id => $_->quantity } @products,
);

ShoppingCart->new(
    user => $user,
    ( map { $_->id => $_->quantity } @products ),
);

use CGI;
use Data::Dumper;
...
    foo => $q->param('foo'),
    bar => 1,
    ];

foo => scalar $q->param('foo'),

ShoppingCart->new(
    user => $user,
    map { $_->id => $_->quantity } @products,
    bank_account => 99,
);