#!/usr/bin/perl -l
use warnings;
use strict;

sub recurse($);

sub recurse($) {
    my $a = $_[0];
    print ">> '$a'";
    $a =~ s/ ^ (.) (.+) (.) $ / join '', $1, recurse $2, $3 /ex;
    print "<< '$a'";
    " ( $a ) ";
}

print "Result: ", recurse 'aaaaaaaaaa';