So it seems that there's two ways your terminal can send "meta" key combinations.

One method is to send escape + the character. So hitting ALT+J will send an escape character followed by 'J'.
Alternatively, your terminal might simply read the character value in and set it's most significant bit high before sending that byte to the terminal. What does does that mean?
Let's say you were to hit ALT+J this time. J, in ascii is 74 in ascii. This number represented in binary is: 01101010. Having alt held down sets the left-most (most significant) bit high, becoming 11101010.

Many terminals allow you to set which method of input it uses. XTerm, for instance, allows you through the menu accessed by control+leftclicking on the terminal (anywhere). Select "Meta sends Escape" and alt-key combinations will send escape+character. If "Meta sends Escape" is unchecked, then your terminal will set-high the most significant bit before sending the character.

Here's a perl oneliner that'll let you see what's going on. The output for mat is: char / ascii / binary where ascii is the hexadecimal value of what you entered, and binary is the binary representation. You'll notice that if "Meta sends Escape" is unchecked, then your terminal will set-high the most significant bit before sending the character.

#!/usr/bin/perl

system("stty raw -echo"); 
while (sysread(STDIN,$foo,1)) {
        printf("%s / %2x - ", ($foo =~ /[[:print:]]/ ? $foo : " "), ord($foo)); 
        print unpack("B32", pack("n", ord($foo))) . "\r\n" 
}