First a tiny nit. substr('p',0,1); is better written as 'p' ;-)

Next there is a potential problem. Your code works correctly only if the AM/PM is the last thing in the string. This may seem to be an unimportant restriction, but imagine the format was 'YYYY/MM/DD hh:mm pp|DD.MM.YYYY hh:mm pp'. In this case the indexes in @order will be incorrect!

The solution is to process the 'pp' at the same time as the other characters:

use strict;
sub prepareFormat {
    my $format = shift;

    # TODO: check that only valid characters appear in the format
    # The logic should be: for any character A-Z in the format string,
    #   die if it's not one of: Y M D h m s p

    my ($i, @order) = 0;
    my $num_chars  = 'YMDhms';
    $format =~ s{([$num_chars]+|pp)(\?)?}{
        $order[$i++] = substr($1,0,1);
        if ($1 eq 'pp') {
            "(AM|PM|am|pm)"
        } else {
            '('.('\d' x length($1))."$2)"
        }
    }ge;

    $format = qr/^(?:$format)$/;
    return [$format, \@order];
}

sub parseDate {
    my ($format, $date) = @_;
    my @data = ($date =~ $format->[0])
        or return;
    my %result;
    for(my $i = 0; $i <= $#data; $i++) {
        $result{$format->[1]->[$i]} ||= $data[$i];
    }

    $result{h} += 12 if (uc($result{p}) eq 'PM' and $result{h} != 12);
    $result{h} = 0   if (uc($result{p}) eq 'AM' and $result{h} == 12);


    return map $result{$_}, qw(Y M D h m s);
}
...
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Jenda
Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.
   -- Rick Osborne

Edit by castaway: Closed small tag in signature