I rewrite your code without use List::Util qw/ max /: 
#!/usr/bin/perl -w
use strict;
my @rows; 
push @rows, [split (/\s/, $_)] while (<DATA>);
my @nn = sort{$a <=> $b} map { @$_- 1 } @rows ;
my $max_cols = pop @nn;

for my $r (0..$max_cols) {
   for my $c (0..$#rows) { 
		$_ = $rows[$c]->[$r] ;
		print if defined;
   }
   print "\n";
}


__DATA__
A B C F G
u b 
1 T A R S 2 P
but I don't understand why this:
my $max_cols = pop sort{$a <=> $b} map { @$_- 1 } @rows ;
doesn't work and I must do this: 
my @nn = pop sort{$a <=> $b} map { @$_- 1 } @rows ;
my $max_cols = pop @nn;

In reply to Re^2: Transpose a matrix of chars by dirac
in thread Transpose a matrix of chars by dirac

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • 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! —
  • 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:
    & &amp;
    < &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.