I think that one of the things that's mildly confusing in this area is that perl does an implicit "local" for you whenever it sets $_ for you (in foreach, map and grep loops), but if you do the setting of $_ yourself, then you've got a new problem you don't see otherwise.

(I still don't understand why our "standard practice" doesn't include doing a "local $_" at the beginning of every sub... but then on the other hand, I guess it isn't all that common to get burned by $_ problems.)

Anyway, here's a script that demos what I'm talking about:

use warnings;
use strict;
use Test::More qw(no_plan);

# we're going to try doing various things to $_,
# and we want to see if it changes the initial value
my $initial_value = 'Some Value';
my @initial_array = qw( wun tew thuree foah fahv sex sevhun );

{#1 map  perlfunc: "locally setting $_ to each element"?
  my $testcase = "map";
  my @array = @initial_array;
  $_ = $initial_value;
  my @whateva = map{ s/^f/F/ } @array;

  is( $_, $initial_value, $testcase);
}

{#2 foreach
  my $testcase = "foreach";
  my @array = @initial_array;
  $_ = $initial_value;
  my @whateva = ();
  foreach (@array) { s/^f/F/; push @whateva, $_ };
  is( $_, $initial_value, $testcase);
}



{#3 while - this one fails
  my $testcase = "while";
  my @array = @initial_array;
  $_ = $initial_value;
  my @whateva = ();
  while (@array) { $_ = pop @array; s/^f/F/; push @whateva, $_};
  is( $_, $initial_value, $testcase);
}


{#4 while with local
  my $testcase = "while with explicit local";
  my @array = @initial_array;
  $_ = $initial_value;
  my @whateva = ();
  while (@array) { local $_ = pop @array; s/^f/F/; push @whateva, $_};
  is( $_, $initial_value, $testcase);
}


{#5 grep
  my $testcase = "grep";
  my @array = @initial_array;
  $_ = $initial_value;
  my @whateva = grep{ m/^f/ } @array;
  is( $_, $initial_value, $testcase);
}
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