The reason it was printing in numbers is because you weren't printing it in scalar context. Try:

$time = scalar localtime;
$day = substr($time, 0,3);

print $day;
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If it's printed without scalar context, it gives a list of values in the format:

($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst)
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See the documentation page on localtime() for more info.