runtime fix for OpenBSD

From Laurent Cheylus, thanks

Makefile

@@ -3,6 +3,7 @@ PREFIX=/usr/local
 test:
 	./impermanence.pl -v -t tests/impermanence.yml
 	-./impermanence.pl -t tests/errored.yml
+	-./impermanence.pl -t tests/size_undefined.yml
 
 install:
 	install -o root -g wheel -m 555 impermanence.pl ${PREFIX}/bin/impermanence

impermanence.pl

@@ -12,6 +12,8 @@ use File::Path qw(make_path);
 use Sys::Syslog qw(:standard :macros);
 use 5.010;
 
+my $isOpenBSD = ($^O eq 'openbsd') ? 1 : 0;
+
 my $verbose = 0;
 
 # display usage and quit
@@ -258,6 +260,9 @@ sub main {
 
     # read file and sanitize content
     $data = read_yml_file_to_struct($configuration_file);
+    if ( ! defined $data->{size} || $data->{size} eq 0 ) {
+        trap_error("size not defined in configuration file");
+    }
     $data = remove_transclusion($data);
 
     # display result and stop if in test mode