use strict;
use warnings;
sub print_response;
my $response_code = 200;
my $response_content = "All's well!";

use constant OK                      =>   0;
use constant BAD_INPUT               =>   1;
use constant SERVICE_ACCESS_ERROR    =>  31;
use constant SERVICE_EXECUTION_ERROR =>  32;
use constant SERVICE_ERROR           =>  30;
use constant UNDEFINED               => 255;

$_ = $response_code;
my $return_status
    = /200/ ?  OK
    : /400/ ?  BAD_INPUT
    : /403/ ?  SERVICE_ACCESS_ERROR
    : /500/ ?  SERVICE_EXECUTION_ERROR
    : /503/ ?  SERVICE_ERROR
    :          UNDEFINED
    ;

print_response $response_content if $return_status == OK;
exit $return_status;
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The constants may be overkill, but if the names are important enough to include as comments, why not make them part of the actual perl code?

UPDATE -- Corrected sense of if statement