$key = @...[...];    <--- Wrong
^      ^                  It may work because Perl is "smart",
|      |                  but Perl doesn't always guess correctly.
+------+

$key = $...[...];    <--- Right
^      ^
|      |
+------+
[download]

A hash key is a scalar, so you need a "$ expression".

# Wrong:
print "hope 1 @{$months->[$i]}\n";
print "hope 2 $monthhash{trim(@{$months->[$i]})}\n";
print "hope 2A $monthhash{@{$months->[$i]}}\n";

# Right:
print "hope 1 $months->[$i][0]\n";
print "hope 2 $monthhash{trim($months->[$i][0])}\n";
print "hope 2A $monthhash{$months->[$i][0]}\n";
[download]

You're assuming an array of one element (@array) is interchangeable with the element ($array[0]). That's not always the case. In fact, you should assume it's not the case.

By the way, @out == 1 is a bug. You should replace @out == 1 with wantarray. You'll get some interesting results when you do.