open my $in_fh1, '<', $in_file1;
open my $in_fh2, '<', $in_file2;
open my $out_fh, '>', $out_file;

while (defined(my $line1 = <$in_fh1>)) {
    print {$out_fh} $line1;
    unless ($. % 10) {
        my $line = <$in_fh2>;
        print {$out_fh} $line;
    }
}