Maybe something like

#!/usr/bin/perl

use strict;
use warnings;

use POSIX;

my @data = qw( 9.90 9.91 9.9111 9.94999 9.95 9.950001 9.96 9.99 9.9999
+9);

foreach my $x (@data) {
    printf "%s => %.2f\n", $x, floor( 20*$x) /20, "\n";
}
[download]

which outputs

9.90 => 9.90
9.91 => 9.90
9.9111 => 9.90
9.94999 => 9.90
9.95 => 9.95
9.950001 => 9.95
9.96 => 9.95
9.99 => 9.95
9.99999 => 9.95

would fit too ?

Your wanted writeoff function is then simply floor( 20 * x) /20.