G'day brassbin,

If we knew what problem you were attempting to solve with this requirement, you'd probably get better answers.

If you moved the checking of 3 values (i.e. s{3}) from GetOptions() to sub1() and passed --test a string containing the values, you could do something like this:

$ perl -Mstrict -Mwarnings -le '
    use Getopt::Long;
    sub sub1 {
        my ($name, $value) = @_;
        my $want = 3;
        my @got = split /\s+/ => $value;
        die "--$name requires $want values. Got: @got\n" unless @got =
+= $want;
        print "$name $_" for @got;
    }
    GetOptions( "test=s" => \&sub1 );
' -- --test 'a b c'
test a
test b
test c
[download]

And with the wrong number of values:

$ perl -Mstrict -Mwarnings -le '
    ...
' -- --test 'a b'
--test requires 3 values. Got: a b
[download]

$ perl -Mstrict -Mwarnings -le '
    ...
' -- --test 'a b c d'
--test requires 3 values. Got: a b c d
[download]

Additional Information Update: You wrote "as if the sub was called 3 times". It's not "as if", that's exactly what's happening. This shows each separate call with the individual arguments highlighted:

$ perl -Mstrict -Mwarnings -E '
    use Getopt::Long;
    sub sub1 { state $i = 0; ++$i; say "$i: >>>$_<<<" for @_ }
    GetOptions( "test=s{3}" => \&sub1 );
' -- --test a b c
1: >>>test<<<
1: >>>a<<<
2: >>>test<<<
2: >>>b<<<
3: >>>test<<<
3: >>>c<<<
[download]