Example Perl script t1.pl:

use strict;
sub dump_user_details {
   my $uid = shift;
   my ($user, $u, $pgid, $home, $shell) = (getpwuid($uid))[0,2,3,7,8];
   print "ruid='$<' euid='$>' uid='$uid' u='$u' pgid='$pgid'\n";
   print "user='$user' home='$home' shell='$shell'\n";
   my @gids = ( $pgid );
   while (my ($name, $pw, $gid, $members) = getgrent) {
      push(@gids, $gid) if grep($_ eq $user, split(' ', $members));
   }
   endgrent();
   print "gids:@gids:\n";
}
my $arg1 = shift;
print "arg1='$arg1'\n";
dump_user_details($<);
dump_user_details($>) if $> != $<;