$cmd = qq{cat <<-ENDOFMESSAGE \none \ntwo \nthree \nENDOFMESSAGE};  
print ($cmd =~ s/\n/\\n/gr);
print "\n";
# cat <<-ENDOFMESSAGE \none \ntwo \nthree \nENDOFMESSAGE
print `$cmd`;
# one 
# two 
# three 

#!/usr/bin/perl
use 5.020;
use warnings;

package PerlIO::via::NoLF {
    sub PUSHED {
        my $self= '';
        bless \$self, shift;
    }

    sub FILL {
        my ($self, $fh) = @_;
        return defined $fh ? nolf(<$fh>) : undef;
    }

    sub WRITE {
        my ( $self, $text, $fh ) = @_;
        return 0 unless defined $text;
        return print $fh nolf($text);
    }

    sub nolf {
        $_[0] =~ s/\n(?!$)/\\n/gr;
    }
}

binmode STDOUT,':via(NoLF)';

my  $cmd = qq{cat <<-ENDOFMESSAGE \none \ntwo \nthree \nENDOFMESSAGE};
print "Ausgabe am Bildschirm soll so auschauen - alles in einer Zeile:\n"; 
print $cmd;
print "\n";
# cat <<-ENDOFMESSAGE \none \ntwo \nthree \nENDOFMESSAGE
print "Ausgeführt muss es aber dann so werden:\n";
print `$cmd`;
# one 
# two 
# three