Ignore what others say about @INC and PERL5LIB. It's not a problem loading Class::ReturnValue.

The docs say:

Returns a false value upon failure to open or parse the file or data; this false value is a Class::ReturnValue object and can be queried as to its error_message.

so

my $cal = Data::ICal->new(filename => 'Calendar.ics');
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should be

my $cal = Data::ICal->new(filename => 'Calendar.ics');
die "Can't open calendar: ".$cal->error_message()."\n" if !$cal;
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I bet you'll the problem is that your script can't find Calendar.ics because the current directory isn't what you assume it to be.