use strict;
use URI::Escape;

while (my $string = <DATA>) {
    chomp $string;
    if ($string eq uri_unescape($string)) {
        print "'$string' was not URI-encoded.\n";
    }
    else {
        print "'$string' was URI-encoded.\n";
    }
}
__DATA__
A%20horse%2C%20a%20horse!%20My%20kingdom%20for%20a%20horse!
A plain string