• 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
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s{\.[0-9]\K([0-9])}{int($1/5)*5}e;
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s{(.*)}{sprintf("%.2f", int($1*20)/20)}e;
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$_ = sprintf("%.2f", int($_*20)/20)        . /(?!)/; # oblig regex
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