You aren't the first to be bitten by this and you won't be the last. This is enough of a "gotcha" that it gets own discussion in perlvar. Because $_ gets assigned the current value foreach loops and functions like grep and map, it is easy to think that each code block gets its own value of $_.

However, $_ is a global variable and it only gets its own value inside a code block if you localize it. The for/foreach loop and functions like grep and map do this implicitly. Any subroutine you write has to do this explicitly, like this:

sub func {
  local $_;   # THIS IS THE KEY LINE YOU NEEDED
  open(READ, "< test.txt");
  while(<READ>) {
    #   print;
  }
  close READ;
  return;
}

# outputs the final line w/o a problem
Files: ree ree1 ree2
$VAR1 = [
          'ree',
          'ree1',
          'ree2'
        ];
The file is ree
The file is ree1
The file is ree2
Files: ree ree1 ree2
[download]

The behavior of $_ can be confusing, but one *nice* thing about it is that you can use it to create your very own special funky map and grep routines. Here is an example of a small routine that lets you do custom processing on every other list member:

use strict;
use warnings;

sub forEveryOther(&@) {
  my @aData = @_[1..$#_];
  my @aResults;

  local $_;
  for (my $i=0; $i < $#aData; $i+=2) {
    $_ = $aData[$i];
    push @aResults, $_[0]->();
  }
  return @aResults;
}
[download]

which we can use like this

my @people = (bob => 53, peter => 200);
my @hello = forEveryOther { "Hello, $_\n" } @people;
print @hello;

# outputs
Hello, bob
Hello, peter
[download]

Best, beth

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