• You're trying to use a regex pattern where a string literal is expected.
• You're think a regex pattern generates the strings it can match in some circumstances.

Some solutions:

s/\.[0-9]\K(?:[0-4]|([5-9]))/$1?5:0/e
[download]

s{\.[0-9]\K([0-9])}{int($1/5)*5}e;
[download]

s{(.*)}{sprintf("%.2f", int($1*20)/20)}e;
[download]

$_ = sprintf("%.2f", int($_*20)/20)        . /(?!)/; # oblig regex
[download]

#!/usr/bin/perl --

use strict;
use warnings;

use Test::More qw( no_plan ); 

Main(@ARGV);
exit(0);

sub writeoff {
    my ( $prefix, $last ) = shift()=~ /^(.+)(.)$/;
    $last = $last < 5 ? 0 : 5;
    return "$prefix$last";
}

sub Main {
    my @input = qw( 9.91  9.92  9.95 9.99  );
    my @wanted_output =  qw( 9.90  9.90  9.95 9.95  );
    my @got = map  { writeoff($_) } @input;

    is_deeply( \@got, \@wanted_output, "@wanted_output" );
}
__END__

ok 1 - 9.90 9.90 9.95 9.95
1..1

[download]

#!/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]

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