To complete the story on GrandFather's explanation, the older 3.05 version of CGI.pm provided the script name this way:

sub script_name {
    return $ENV{'SCRIPT_NAME'} if defined($ENV{'SCRIPT_NAME'});
    # These are for debugging
    return "/$0" unless $0=~/^\//;
    return $0;
}
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The 'debugging' code is not present in the 3.59 version - it relies exclusively on the environment variable.