I'm prepared to bet the error you got was: Can't use string ("3") as a SCALAR ref while "strict refs". This is one reason why your function isn't very friendly.

What your function is doing is _expecting_ a reference, then modifying the value referenced. So you can call it by:

foo ( \$data );
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Action at a distance like that is generally considered bad form - you don't know what what the subroutine "foo" is doing to your local variable. So normally, what you would expect to pass in a value, and get back a result.

You should probably note - you _do_ get a result back from your subroutine, that's the initial (pre-increment) value of $data. That can be confusing

Here are some guesses at what the proprietary code you have may resemble (more or less). Two have been discussed,  bar() is new. All are valid constructs in the sense that they are not deprecated as of 5.14. Whether they are good ideas or not is another question. Please pick one of these as the closest to what you have:

c:\@Work\Perl>perl -wMstrict -le
"print qq{perl version $]};
 ;;
 sub foo {
   my ($data) = @_;
   $$data++;
   }
 ;;
 my $x = 3;
 foo(\$x);
 print $x;
 ;;
 sub bar (\$) {
   my ($data) = @_;
   $$data++;
   }
 bar($x);
 print $x;
 ;;
 sub bof {
   $_[0]++;
   }
 bof($x);
 print $x;
"
perl version 5.008009
4
5
6
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