Round the string

Perl, 22 20 bytes

printf"%.*f",pop,pop

Using:

perl -e 'printf"%.*f",pop,pop' 123456789.987654321 3

It is Dada’s version of code. Previous:

printf"%*2\$.*f",@ARGV

PHP, 33 31 bytes

PHP rounds correctly too (at least on 64 bit):

printf("%.$argv[2]f",$argv[1]);

takes input from command line arguments. Run with -r.

PHP, no built-ins, 133 bytes

[,$n,$r]=$argv;if($p=strpos(_.$n,46))for($d=$n[$p+=$r],$n=substr($n,0,$p-!$r);$d>4;$n[$p]=(5+$d=$n[$p]-4)%10)$p-=$n[--$p]<"/";echo$n;

Run with -nr or test it online.

breakdown

[,$n,$r]=$argv;             // import arguments
if($p=strpos(_.$n,46))      // if number contains dot
    for($d=$n[$p+=$r],          // 1. $d= ($r+1)-th decimal 
        $n=substr($n,0,$p-!$r); // 2. cut everything behind $r-th decimal
        $d>4;                   // 3. loop while previous decimal needs increment
        $n[$p]=(5+$d=$n[$p]-4)%10   // B. $d=current digit-4, increment current digit
    )
        $p-=$n[--$p]<"/";           // A. move cursor left, skip dot
echo$n;

A null byte doesn´t work; so I have to use substr.

Python, 114 105 103 96 91 89 bytes

Saved 5 bytes thanks to Kevin Cruijssen
Saved 2 bytes thanks to Krazor

from decimal import*
d=Decimal
lambda x,y:d(x).quantize(d('0.'[y>0]+'1'*y),ROUND_HALF_UP)

Try it online!