use CGI;
my $q = CGI->new;
use Data::Dumper;
%cgi_vars = (
    a => 1,
    fname => '',
);

my $q = CGI->new();
## This worked
if ($cgi_vars{fname}) {
    $q->param('fname' => $cgi_vars{fname});
} else {
    $q->param('fname' => "");
}

print Dumper($q->param('fname'));

$q = CGI->new;
## But this did not
$q->param('fname' => $cgi_vars{fname});

print Dumper($q->param('fname'));
__END__
$VAR1 = '';
$VAR1 = '';