Try the following with and without the 'local $_;' line and you'll see what he's talking about.

use strict;

sub getfile {
local $_;
    my $filename = shift;
    local *F;
    open F, "< $filename" or die "Couldn't open `$filename': $!";
    my $contents;
    while (<F>) {
      s/#.*//;              # Remove comments
      next unless /\S/;     # Skip blank lines
      $contents .= $_;      # Save current (nonblank) line
    }
    return $contents;
}

my @array = ("base_tester", "Foo.pm");
foreach (@array) {
  my $filename = $_;
  my $f = getfile($filename);
}

print "@array\n";
[download]

That's what I tried to see the effects, and I was rather surprised.