First off, if you want the replacement part of a substitution operation interpreted as perl code, use the /e switch. Second, if you have a reference to a subroutine, use $ref->() to call it.

Third, don't use integer numbers for ASCII characters -- just use the characters instead.

Check this out:

use strict;
use warnings;

my @CHARS = ('A'..'Z', 'a'..'z');

sub rand_char {
    return $CHARS[rand @CHARS];
}

while (<DATA>) {
        s/\w/rand_char()/eg;
        print $_;
}
__DATA__
ABCDEFG
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