use strict;
use warnings;

my @products = (
    {id => 'jam', quantity => 10},
    {id => 'cream', quantity => 4},
    );

mySub (user => 'Flibble',  map {$_->{id} => $_->{quantity}} @products 
+);
print "\n";
mySub (user => 'Flibble', (map {$_->{id} => $_->{quantity}} @products)
+);

sub mySub {
    my %params = @_;

    print "$_: $params{$_}\n" for keys %params;
}
[download]

Prints:

cream: 4
user: Flibble
jam: 10

cream: 4
user: Flibble
jam: 10
[download]

and the difference is?