On the off chance that you are more interested in the current group membership of the current running process, there's POSIX::getgroups.

use POSIX qw(getgroups);
# get list of the current groups for this process
my @gidlist=getgroups();
# you'd  have to translate numeric gid's to 
# the names here
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Update:

Here's another shot at getting secondary groups, this one from the id command in the Perl Power Tools project:

while ( my($name,$pw,$gid,$members) = getgrent ) {
  push(@rgids,$gid) if ( grep($_ eq $user,split(/\s+/,$members)) );
}
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