Perl failing to create cgi session - perl

Question

I have the code as shown below. A BEGIN which loads the session or, if none is yet created, it creates one. But it doesn't do it all the time. It's a login script; If I enter the PIN and it's wrong, the script displays the login form again, which is submitted to this same script. Up to 3 attempts permitted but, it will fail to load the session, usually on attempt 2. Inconsistent so, please can anyone see what might be wrong and why is the session sometimes not loading.
I do have warnings enabled and I have shown that in the code.
I used to start the script with 'print $session->header' but, having changed to 'print $cgi->header;' I can see clearly that the session is undefined, when the script fails. I should add that, if I refresh the failed page perhaps as many as 5 times, the session does eventually reload with all data intact.
#!/usr/bin/perl
#use CGI::Carp qw/warningsToBrowser fatalsToBrowser/;
use strict;
use warnings 'all';
use CGI qw(:all);
use CGI::Session;
use Crypt::PasswdMD5;
use DBI;
use Data::Dumper;
my $cgi = CGI->new;
my $session;
my $sessions_dir_location;
my $session_id;
BEGIN{
unshift #INC, "/var/www/vhosts/example.com/subDomain.example.com/cgi-bin/library";
my $document_root = $ENV{'DOCUMENT_ROOT'};
$document_root =~ s/\///;
my ( $var
, $www
, $vhosts
, $domain
) = split ('/', $document_root, 5);
$sessions_dir_location = '/' . $var . '/' . $www . '/' . $vhosts . '/' . $domain;
$session = CGI::Session->load() or die CGI::Session->errstr();
if ( $session->is_expired ) {
print $session->header(),
$cgi->start_html(),
$cgi->p("Your session timed out! Refresh the screen to start new session!"),
$cgi->end_html();
exit(0);
}
if ( $session->is_empty ) {
$session = new CGI::Session(undef, undef,
{Directory=>"$sessions_dir_location/sessions_storage/"}) or die CGI::Session->errstr;
}
#add the library dir to #INC;
use lib do {
use Cwd 'realpath';
my ($dir) = __FILE__ =~ m{^(.*)/};
realpath("$dir/library");
};
use feature 'say';
use FindBin '$RealBin';
use lib $RealBin;
use lib "$RealBin/library";
}
my $self = $cgi->url;
my %login = $cgi->Vars;
print $cgi->header;
# capture and display warnings
local $SIG{__WARN__} = sub {
my $message = shift;
print $cgi->header;
print qq($message);
};
print qq(<pre>);
print Dumper \%login;
print qq(</pre>);
print qq(<pre>session);
print Dumper \$session; #undef
print qq(</pre>);
#next is line 141
my $session_stored_user_name = $session->param("entered_user_name");
Error message is this:
Can't call method "param" on an undefined value at /var/www/vhosts/example.com/subDomain.example.com/cgi-bin/dashboard-login/login-with-pin.pl line 141, <DAT> line 45.
Please, also, what or where is <DAT> line 45?