Not sure what you might've been expecting? I threw lots of prints at your code:

  $pattern = '[aeiou]';
  ( $s1, $s2, $s3, $s4 ) = qw( name nerd noodle froodle );

  printf "was:  '%s'\n", join("'   '",$s1,$s2,$s3,$s4);
  foreach my $item ($s1,$s2,$s3,$s4,) {
    $item =~ s/$pattern/ /g;
  }
  printf "now:  '%s'\n", join("'   '",$s1,$s2,$s3,$s4);

  ( $s1, $s2, $s3, $s4 ) = qw( name nerd noodle froodle );
  @a = ( $s1, $s2, $s3, $s4 );

  print "\n";
  printf "was:  '%s'\n", join("'   '",$s1,$s2,$s3,$s4);
  printf "was:  '%s'\n", join("'   '",@a);
  foreach my $item (@a) {
    $item =~ s/$pattern/ /g;
  }
  printf "now:  '%s'\n", join("'   '",$s1,$s2,$s3,$s4);
  printf "now:  '%s'\n", join("'   '",@a);
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and got

  was:  'name'   'nerd'   'noodle'   'froodle'
  now:  'n m '   'n rd'   'n  dl '   'fr  dl '

  was:  'name'   'nerd'   'noodle'   'froodle'
  was:  'name'   'nerd'   'noodle'   'froodle'
  now:  'name'   'nerd'   'noodle'   'froodle'
  now:  'n m '   'n rd'   'n  dl '   'fr  dl '

When you did

  my @look_for = ($name,$nerd,$noodle,$froodle);
[download]

you copied the _values_ in each variable into the array. You then looped through the array elements changing their values through the magic of foreach aliasing. But that doesn't change the variables you copied from.