One line, using a substitution wrapped in a while loop and no split. Obviously, you can remove the print statements with no ill effect. A split-base solution as offered by Yary and ikegami is probably clearer and more maintainable. This algorithm assumes you can easily differentiate your keys from your values, as in your example. 
#!/usr/bin/perl
use strict;
use warnings;

my $n_terms = 5;
my $string = "";
my $letter = "A";
$string .= $letter++ . " # " for (1 .. $n_terms);
$string .= join " # ", 1 .. $n_terms;

print "Input:        $string\n";

while ($string =~ s/((?:^|[0-9]+\s*\#\s*)[A-Z]+\s*\#\s*)
                    ([^0-9]+?\#\s*)
                    ([0-9]+\s*\#\s*)
                    /$1$3$2/x) {
    print "Intermediate: $string\n";
};

print "Output:       $string\n";
__END__
Input:        A # B # C # D # E # 1 # 2 # 3 # 4 # 5
Intermediate: A # 1 # B # C # D # E # 2 # 3 # 4 # 5
Intermediate: A # 1 # B # 2 # C # D # E # 3 # 4 # 5
Intermediate: A # 1 # B # 2 # C # 3 # D # E # 4 # 5
Intermediate: A # 1 # B # 2 # C # 3 # D # 4 # E # 5
Output:       A # 1 # B # 2 # C # 3 # D # 4 # E # 5

[download]

In reply to Re: rearranging matched parts of a string while using /g and without using split by kennethk
in thread rearranging matched parts of a string while using /g and without using split by jbullock35

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.