From the docs for Getopt::Long

Using a colon : instead of the equals sign indicates that the option value is optional. In this case, if no suitable value is supplied, string valued options get an empty string '' assigned, while numeric options are set to 0 .

C:\Users\Bill\forums\monks>type perl_script.pl
use strict;
use warnings;
use Getopt::Long;

my $temp;
GetOptions("flag:s" => \$temp);
my @flags = split /,/, $temp ;
print join(' ', @flags,), '     ', scalar @flags, " strings\n";


C:\Users\Bill\forums\monks>.\perl_script.pl --flag
     0 strings

C:\Users\Bill\forums\monks>.\perl_script.pl --flag a
a     1 strings

C:\Users\Bill\forums\monks>.\perl_script.pl --flag a,b,c
a b c     3 strings
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For me, the use case seems too DWIM. I could be wrong and just can't think of an ideal case but it seems like you're trying to overload an argument list to be a trigger as well. It might be better served by either a default noop trigger arg like -flag 0 or -flag . or just having a new boolean flag, like -runnit, to do what you want -flag without arguments to do. You also have the option of processing your own argument list after (or even before but after is much more sane) Getopt is done if its options don't cover exactly what you want.

It says :-

Often it is useful to allow comma-separated lists of values as well as
multiple occurrences of the options. This is easy using Perl's split()
+ and
join() operators:

        GetOptions ("library=s" => \@libfiles);
        @libfiles = split(/,/,join(',',@libfiles));
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