Module code should localize any special variables it uses in each location it uses them. This is a smart and sane practice. Watch:

package MyConfigReader;

sub readConfig {
  my ($file,$var) = @_;
  open FILE, "< $file" or die "can't read $file: $!";
  while (<FILE>) {
    chomp;
    my ($k,$v) = split /=/, $_, 2;
    return $v if $k eq $var;
  }
  close FILE;
  return;
}
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That looks innocent, right? Watch it get blown to hell:

use MyConfigReader;

$/ = "not gonna happen";
MyConfigReader::readConfig("whatever.dat");
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My program has just modified the input record separator variable. The module relied on that being \n, and now it isn't.

sub readConfig {
  my ($file,$var) = @_;
  local ($_, $/);
  $/ = "\n";
  # rest of function
}
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So yes, this is something you should be doing. Trusting the user is potentially silly. Perhaps there should be a switch like -T that catches blind use of "true globals" like you demonstrated.

japhy -- Perl and Regex Hacker