use Getopt::Long qw(GetOptions);
Getopt::Long::Configure(qw(posix_default no_ignore_case));

GetOptions(\my %OPT,
    "mandatory!",
);

die "Option --mandatory not specified.\n"
  unless exists $OPT{mandatory};

print "OK.\n";

##</code><code>##

$ testcmd.pl
Option --mandatory not specified.

$ testcmd.pl --manda
Unknown option: manda
Option --mandatory not specified.

$ testcmd.pl --mandatorY
Unknown option: mandatorY
Option --mandatory not specified.

$ testcmd.pl --mandatory
OK.

$ testcmd.pl --nomandatory
OK.