my $datei1 = '/path/to/file1.txt';
my $datei2 = '/path/to/file2.txt';
my $output = '/path/to/file3.txt';

my @lines;

tie my @lines_file1, 'Tie::File', $datei1 or die $!;
tie my @lines_file2, 'Tie::File', $datei2 or die $!;

die "ungleiche Länge" if scalar(@lines_file1) != scalar(@lines_file2);

for (0..$#lines_file1 ){
    push @lines, $lines_file1.' '.$lines_file2;
}

untie @lines_file1;
untie @lines_file2;

open my $out, '>', $output or die $!;
print $out $_,"\n" for @lines;
close $out or die $!;