I really appreciate two tricks:

• s// /
  I understand this, and have used the empty expression before with m//, but never thought to try it with s///.
   
• for($/)x16
  A slick way of getting the newline (vis-à-vis -l print until...)!
  Changing the loop to for($_)x16 is informative.
  And, since a bare print prints both $_ and $/, I found it rather unobvious which variable holds the strings.

for($/)x16
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A slick way of getting the newline (vis-à-vis -l print until...)! Changing the loop to for($_)x16 is informative. And, since a bare print prints both $_ and $/, I found it rather unobvious which variable holds the strings.

Actually, $/ is the input record separator, so a bare print does not print it. What's being printed is just the (localized) $_ inside the for loop. Of course, a bare print will print $\, aka the output record separator, often seen in golf, as in, for example, The golf course looks great, my swing feels good, I like my chances (Part I).

The for($/)x16 loop aliases $/ and, despite the x16, only the single $/ value is (repeatedly) changed, as you will discover by printing it after the loop has terminated. Replacing "print" with "warn" like so:

s/^|\d+/$&+$'||1/eg,warn("du='$_' ds='$/'\n"),s// /for($/)x16;
warn("at end du='$_' ds='$/'\n");
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should clarify. BTW, this eccentric "for loop aliasing" trick is often seen in golf; see, for example, Drunk on golf: 99 Bottles of Beer in the "Bottle Golf Tip No 2" section (search for Cantor). Finally, the "for loop aliasing" trick is a specific example of Eugene van der Pijll's general golfing mantra, "Can't possibly work, let's try it anyway". :)

Doh, I flubbed that ($/ vs. $\). I so often print$/,/expr/?this:that, I confused myself.

And darn you - I had already read your lengthy linked topics, and perhaps all of their references, but I find them irresistable! This time through, I most enjoyed this gem:

   s///ge>>9?print:do$0
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The :do$0 is one *sick* loop. And the return value from s/// idea gave me the following "for 1 to 15" method:

   perl -le'$_=1;{print,s/(\d+) ?/+$1 $1/g,s/\S+/$&/eeg>16||redo}'
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A couple more tricks for me!

#!/usr/bin/perl -l
print,s/^/_/,s/./$'& z^$&^Z/eg,y/5/o/for($a=o)x80
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With the same idea for Sierpinki:

#!/usr/bin/perl -l
print,s/^/_/,s/./$'& z^$&^Z/eg,y/5/o/for($a=o)x80
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There are a variety of Sierpinski triangle solutions given in the documentation for Acme::EyeDrops. For example, this short one was composed by mtve in 2002:

#!perl -l
$x=2**pop;print$"x--$x,map$x&$_?$"x2:"/\\",0..$y++while$x
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This version takes a single command line argument; running "perl sier.pl 5", for example, produces the same result as that shown in Re: Pascal-Sierpinski for golfers.