The difference between `$/` and `local $/`

Question

For my program, I need to use FASTA files and do some calculations with them. In order to do that, I used local $/ = "^>, to chomp up my file in to header line and sequence lines. While my program does what I want it to do, why can't I just simply use $/ = "^>"? When I tried it, my results were not what I needed, I'm interested why is that so. Here's my simplified code:

my @array;
while(<>){
    local $/ = "^>";
    chomp;
    push (@array, $_);
    if(eof){
        for(@array){
            ...
        }
    ...
    }
    if(eof){
        @array = ();
    }

Answer 1

local $var saves the value of $var, and adds a directive to the stack that will cause the value of $var to be restored when the scope is exited (even by exception). It's the closest thing to my that's available to package variables.

$_ = 123;
{
   local $_ = 456;
   # $_ is 456 here.
}
# $_ is back to being 123 here.

This is useful to avoid causing problems in surrounding code or in the caller (in the case of subs).

Note that value of $/ is matched character-for-character. It's not treated as a regular expression.

Note that $/ appears to be set for no reason in the code you posted (unless you left something out).

---

why can't I just simply use $/ = "^>"

Then, the change wouldn't be undone at the end of the block, so it would affect the <> in the while condition as well as any code after your loop that performs reads.

---

How I'd handle a FASTA file:

my ($header, $seq);
while (1) {
   my $line = <>;
   if (!defined($line) || $line =~ /^>/) {
      work($header, $seq) if defined($header);

      last if !defined($line);

      chomp($line);
      $header = substr($line, 1);
      $seq = "";
   } else {
      chomp($line);
      $seq .= $line;
   }
}