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.

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.