This is actually an exercise in Jeff Friedl's Mastering Regular Expressions.
His final reccomendation is to skip the sprintf alltogether and just use

$percent_change =~ s/(\.\d\d(?>[1-9]?))\d+/$1/
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Which always keeps 2 digits, i.e. 2.500 => 2.50.
If you don't wan that to happen, use

$percent_change =~ s/(\.\d(?>[1-9]{0,2}))\d+/$1/
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