What is the actual error message as produced by perl? The one listed in your post isn't.

What do you get with

use Data::Dump qw/ dd /;
dd( $prn );
[download]

get_unique_spool makes use of a tainted $ENV{TEMP}

This is a web interface and any generated errors are run through CGI::Application::dump().

Public error message: Internal exception error Internal error message:
+ Insecure $ENV{PATH} while running with -T switch

File: /usr/local/share/perl/5.24.1/Printer/Unix.pm
Line: 129

...

Current Run mode: 'transactions_approve'

...

Query Environment:
    CONTEXT_DOCUMENT_ROOT => '/var/www/html'
    CONTEXT_PREFIX => ''
    DOCUMENT_ROOT => '/var/www/html'
    GATEWAY_INTERFACE => 'CGI/1.1'
    HTTP_ACCEPT => '*/*'
    HTTP_ACCEPT_ENCODING => 'gzip, deflate'
    HTTP_ACCEPT_LANGUAGE => 'en-US'
    HTTP_CONNECTION => 'Keep-Alive'
    HTTP_COOKIE => 'CGISESSID=19ba4d2d00de2da0f433055d04e8745b'
...
    MOD_PERL => 'mod_perl/2.0.10'
    MOD_PERL_API_VERSION => '2'
    PATH => '/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/
+bin'
...
[download]

I'm resetting ENV{PATH} and now ENV{TEMP} in Printer.pm, Printer::Unix.pm, and my code. Somehow It's still getting the wrong PATH. It will work once or twice, then fail.