++ for demonstrating the problem in a simple self-contained piece of code. One advantage of reducing a problem to the smallest set of instructions that replicates the problem, is that it allows you to debug it using your Eyeball Mk I (and maybe a pencil and a piece of paper).

Using your sample code, let's Eyeball Debug it, starting at the top and following along with the flow of the code.

I don't know how you pronounce $_ — maybe you call it "dollar underscore" whatever, but I call it "it". That's right. So if ($_ == 3), to me, reads "if it equals 3", and @foo = grep { defined $_ } @bar; I would pronounce as "at-foo now is ... grep-if it is-defined ... from at-bar".

Why do I share this? Because I feel it really helps to think of $_ as being "it". Let's go:

STEP    CODE                            MEANING                       
+      IT IS NOW...
1       &main();                        call 'main', no arguments     
+      undef
2       my @mylist = qw(a b c);         create and define @mylist     
+      undef
3       foreach(@mylist)                iterate over @mylist          
+      undef

    3a                                  iteration 1                   
+      it is now "a"
4       &fun($_);                       &fun(it)                      
+      "a"
5       open(STATUS, "echo d-e-f|");    echo "d-e-f" into new fh STATU
+S     "a"
6       while (<STATUS>)                sequentially read lines       
+      "a"
    6a                                  iteration 1: read it.         
+      it is now "d-e-f"
7       if (/d-e-f/)                    if it matches (and it does),  
+      "d-e-f"
8       close(STATUS)                   close the FH                  
+      "d-e-f"
9       return;                         go back to whence we came     
+      "d-e-f"
10      print "changed: \$_ $_\n"       print it                      
+      "d-e-f"

    3b                                  iteration 2                   
+      it is now "b"
...snip...
[download]

This clearly shows where, how, and why it (or dollar-underscore) got changed.

Solutions have been given elsewhere in this thread. I just wanted to show you how I would've debugged it. The code, that is, not the value of $_. Or maybe both.

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