You have omitted one key piece of information: how are you calling your program?

For example, do you call it with "-t" or with "-t 1"? There is a difference.

I have added some print statements to your code to help debug: 

> cat 714766.pl
#!/usr/bin/env perl

use strict;
use warnings;
use Data::Dumper;
use Getopt::Std;

#print 'ARGV ',Dumper(\@ARGV);

my $test = 1;

my %opt;
getopt('st',\%opt);
#print 'opt ',Dumper(\%opt);

if ($opt{'t'}) {
    $test = 1; ## testing
    #print "set test=1 again\n";
} elsif ($opt{'s'}) {
    $test = 0; ## not testing
    #print "set test=0\n";
}
#print "test=$test\n";

if ( (!(defined($opt{'s'}))) && (!(defined($opt{'t'}))) ) {
    print "neither -t or -s used\n";
}
else {
    print "either -t or -s used\n";
}

[download]

Now some sample program calls (5.8.8 on linux): 

> ./714766.pl
neither -t or -s used
>
> ./714766.pl -t
neither -t or -s used
>
> ./714766.pl -t 0
either -t or -s used
>
> ./714766.pl -t 1
either -t or -s used
>
> ./714766.pl -s 5
either -t or -s used

[download]

Data::Dumper can help you understand what is in %opt

Update: It seems to me that the documentation for Getopt::Std is incorrect:

getopt() and getopts() will also accept a hash reference as an optional second argument. Hash keys will be x (where x is the switch name) with key values the value of the argument or 1 if no argument is specified.
If no argument is specified for getopt, the hash key is NOT set to 1 (Dumper shows it as 'undef').

In reply to Re: Detecting an undefined hash key by toolic
in thread Detecting an undefined hash key by LesleyB

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