Obviously, I'm a little late to this party, but I stumbled across this while checking to see whether I'd have to (re)invent a wheel to answer a question on the Perl Beginners mailing list. This is the solution I ultimately came up with:

#!/usr/local/bin/perl -l
use strict;
use warnings;

my $user = $ARGV[0];
my @gr_list;
my @pw_info;

while (@pw_info = getpwent) {
  if ($pw_info[0] eq $user) {
    my @gr_info;
    while (@gr_info = getgrent) {
      push(@gr_list, $gr_info[2]) if ($gr_info[3] =~ /\b$user\b/);
    }
    push(@gr_list, $pw_info[3]);
  }
}

print foreach @gr_list;
[download]

It's not stunningly terse or clean-looking, at least in my estimation, but it works and doesn't involve calling out to external system utilities.