I had looked at the various ways to call the sub (do I put in & or leave it out), but didn't think in this case it mattered.

It didn't matter. I'm not sure why that was pointed out to you. The basic thing is that, to modify a passed parameter, you need to do it via @_, like

# surround all uppercase characters in the passed parameter with *'s
# and all lowercase characters with _'s
# (modifies parameter in-place)
sub transmogrify {
   my $string = $_[0];
   $string =~ s/([A-Z])/*$1*/g;
   $string =~ s/([a-z])/_$1_/g;
   $_[0] = $string;
   return;
}

$text = "I met this guy, and he looked like he might have been a hat c
+heck clerk at an ice rink, which in fact, he turned out to be.  Let X
+ = X.";
transmogrify($text);
print $text;
[download]

Or you need to not try to modify the parameter, but instead return it, and have the caller call the sub appropriately:

# surround all uppercase characters in the passed parameter with *'s
# and all lowercase characters with _'s
sub transmogrify {
   my $string = $_[0];
   $string =~ s/([A-Z])/*$1*/g;
   $string =~ s/([a-z])/_$1_/g;
   return $string;
}

$text = "I met this guy, and he looked like he might have been a hat c
+heck clerk at an ice rink, which in fact, he turned out to be.  Let X
+ = X.";
$text = transmogrify($text);
print $text;
[download]

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