my $n = 34.57889;
my $na = int($n * 1000) / 1000;
print "$n $na\n";
__OUTPUT__
34.57889 34.578

##</code><code>##

my $n = 34.57889;
my $na = sprintf("%3.4f", $n);
$na =~ s/(?<=\.\d\d\d)\d*$//;
print "$n $na\n";
__OUTPUT__
34.57889 34.578