It's pretty straightforward to construct an SSCCE showing this working just fine both with the specified parameter and without:

$ cat foo.cgi
#!/usr/bin/env perl
use strict;
use warnings;
use CGI::Tiny;

cgi {
    my $cgi = $_;
    my $bookparam = $cgi->param('book') || '';
    $cgi->render (text => "bookparam is $bookparam\n");
}
$ (export QUERY_STRING='book=bar'; ./foo.cgi)
Date: Mon, 12 Dec 2022 13:57:29 GMT
Content-Type: text/plain;charset=UTF-8
Content-Length: 17

bookparam is bar
$ ./foo.cgi 
Date: Mon, 12 Dec 2022 14:01:26 GMT
Content-Type: text/plain;charset=UTF-8
Content-Length: 14

bookparam is 
$

[download]

As others have suggested, it seems that your $cgi is undefined. The error is in the code you haven't shown us.

🦛

In reply to Re: CGI::Tiny versus undefined parameter/form field by hippo
in thread CGI::Tiny versus undefined parameter/form field by Polyglot

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