#!/usr/bin/perl -w
use strict;
use Data::Dumper;

local $/ = '';
my $data = <DATA>;

do {
  $data =~ m|(<Text>.+</Text>)|s;
  my $prepared_string = prepare_string($1);
  $data =~ s/$1/$prepared_string/s;
} while($1);

print Dumper $data;

## subs ##
sub prepare_string {
  my $string = shift;
  $string =~ s/\n|<Text>|<\/Text>//g;
  
  my $chunk_size =  4;
  my $prepared_string;
  for (my $i = 0; $i <= length($string); $i=$i+4)  {
    my $part = substr($string, $i, $chunk_size);
    $prepared_string .= "<Text>$part</Text>\n";
  }
  return $prepared_string;
}

__DATA__
<Feld><Text>Hallo liebe Perl-Gemeinde,
ich wünsche euch ein schönes Fest und
einen guten Rustch ins neue
Jahr.</Text></Feld><Sig>
abc</Sig>