use Encode;

# ... get the uri string into $_ by whatever means ...

$_ = "a%20%C3%A5%20%2F";

# first, let's turn the uri encoded string (with "%HH" for some bytes) into binary:

s/\%([0-9a-f]{2})/chr(hex($1))/egi;

# then, since this produces a utf-8 byte sequence, let's "decode" that into utf-8

$_ = decode( 'utf8', $_ );

# $_ now has utf8 flag set, and contains the string with expected unicode characters

binmode STDOUT, ":utf8";

print;