{
        my @FIELDS = qw( foo bar baz);
        sub set_params
        {
                return { map { $_ => $_ } @FIELDS };
        }
}
my $params = set_params;
use Data::Dumper "Dumper";
print Dumper($params);

__OUTPUT__
$VAR1 = {
          'bar' => 'bar',
          'baz' => 'baz',
          'foo' => 'foo'
        };

##</code><code>##

sub set_params
{
        { map { $_ => $_ } @FIELDS };
}

$ perl -MO=Deparse /tmp/hash.pl
sub set_params {
    {
        map {$_, $_;} @FIELDS;
    }
}

sub set_params
{
        +{ map { $_ => $_ } @FIELDS };
}

$ perl -MO=Deparse /tmp/hash.pl
sub set_params {
    {map({$_, $_;} @FIELDS)};
}

sub set_params
{
        return { map { $_ => $_ } @FIELDS };
}

$ perl -MO=Deparse /tmp/hash.pl
sub set_params {
    return {map({$_, $_;} @FIELDS)};
}