-r (idea)

It seems that -r came from the shell scripting languages. Namely bash and sh. Most of the file testing operators are commong across both perl and bash. Little snippet of code as an example:

#!/bin/bash
echo Checking ${1} permissions...
if [ -r ${1} ] then
    echo File is readable
else
    echo File is not readable
fi

#!/usr/bin/perl
print "Checking $ARGV[0] ...\n";
if (-r $ARGV[0])
{
    print "File is readable\n";
}
else
{
    print "File is not readable\n";
}