use feature 'say';

$user = shift || $ENV{"USER"};
say "User $user belongs to these groups:";
@list = ();
open (my $fh, "grep $user /etc/group |") or die "Error: $!\n";
while ($group = <$fh>) {
    chomp $group;
    $group =~ s/(.*?):.*/$1/;
    push @list, $group;
}
close $fh;
@slist = sort @list;
say "@slist";