use strict;
use warnings;

my $text =   " R12345678-A1234567 INCL.EUR 3,31 MWST JULI MONATL. GEB HR T-DSL FLAT 01.07.04-31.07.04     12,34";

while ( $text =~ / (.{0,27}(?!\S))/g ) {
    my $out = $1 . ( '_' x ( 27 - length( $1 ) ) );
    print "$out\n";
}