Let me presume to answer for tybalt89.

my ($cluster, $member) = split;

This depends on default behavior of split, and is equivalent to
    my ($cluster, $member) = split ' ', $_;
The  ' ' split pattern is a special case explained in split docs.

print $cluster eq $member ? "\n" x ($. > 1) : ', ', $member;

This is a bit more tricksy. From the inside out:

• ($. > 1)  $. is input line counter (update: see perlvar).  ($. > 1) evaluates to either '' (empty string) or 1 and will be 1 for every input line after the first.
• "\n" x ($. > 1) Repeats a newline zero times for the first line of input (empty string silently promoted to 0 in this special case), once for every subsequent input line.
• $cluster eq $member ? Newline_or_Nada : ', ' Ternary expression. If  $cluster eq $member true, output newline for every input line after the first (see previous item); if false, output  ', ' string.
• print Ternary_Expression, $member; print result of ternary expresssion (see previous item), then  $member string.

And that's all there is to it (I think).

Update: Minor wording changes.