> wait this might go wrong if cmp_undef("",undef) returns 0. (Not sure)

actually it works undef is not cast to "" and hence less than empty string°

but it seems you didn't sue strictures, that's why I needed to add no warnings 'uninitialized';

> So you'd need to treat the 3 extra cases for $a, $b being undef and return -1,0,1 accordingly.

see cmp_undef2 for that solution

use strict;
use warnings;
use Data::Dump;

sub cmp_undef1 {
   return $a cmp $b 
     if defined $a and defined $b;
   no warnings 'uninitialized';
   return $b cmp $a;                    # invert order otherwise
}

sub cmp_undef2 {
   if (defined $a) {
      if (defined $b) {
         $a cmp $b;
      } else {
         -1
      }
   }
   else {
      if (defined $b) {
         1
      } else {
         0
      }
   }
}



my @array =( (undef)x 3, qw/c b a/, ("")x3);

my @result = sort cmp_undef1 @array;

dd \@result;

@result = sort cmp_undef2 @array;

dd \@result;
[download]

output

["", "", "", "a", "b", "c", undef, undef, undef]
["", "", "", "a", "b", "c", undef, undef, undef]
[download]

I don't want to elaborate on the sort SUBNAME LIST syntax see sort for details and other variations. (TIMTOWTDI I chose the shortest)

°) which is strange because

  DB<1> p "" eq undef
1
[download]

---

---

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.