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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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