my $New_mo;

# ==, not =
if ($Month == 0)
{
    # no my!
    $New_mo = 'Jan';
}
if ($Month == 1)
{
    $New_mo = 'Feb';
}
# ...

##</code><code>##

my @monthNames=qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct
                  Nov Dec);
my $New_mo = $monthNames[$month];