The int() routine will truncate to an integer value, "toward zero." So int( $x * 100 ) / 100 will truncate to the penny.

my $n = 34.57889;
my $na = int($n * 1000) / 1000;
print "$n $na\n";
__OUTPUT__
34.57889 34.578
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A slower but interesting alternative approach... if you're dealing with long strings of digits, is to render the number to MORE digits than required, then strip the undesired digits.

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
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Take care when you're talking about "truncate", "floor", "ceiling" and "rounding" when you deal with negative numbers... make sure you're defining your problem correctly, too.