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

my $text = ' a b c d a b c d hallo';

my $trace = $text;
my $offset = 0;

$text =~ s{(a)}{substitute($1)}ge;
print ">>$text\n";

sub substitute {
    my ($vorher) = @_;

    my $nachher = 'aa';

    substr($trace, $offset + $-[0], length($vorher), $nachher);
    print ">$trace\n";

    $offset += length($nachher)-length($vorher);

    return $nachher;
}