When perl compiles your program, any constant regular expressions in the code (ie any that don't interpolate any variables) are compiled. Any that do interpolate variables will be compiled once when they are first used, and on further uses are re-compiled only if they have changed since the last use.

So in this code, the regular expression is compiled only once (at compile time):

  for (@words) {
    print if /foo/;
  }
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And also in this code (but at runtime):

  my $s = "foo";
  for (@words) {
    print if /$s/;
  }
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In this code, the two regular expressions are each compiled only once:

  my($s1, $s2) = qw/ foo bar /;
  for (@words) {
    print if /$s1/;
    print if /$s2/;
  }
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But in this code, because the one regular expression is being used alternately interpolated with "foo" and "bar", it will be recompiled each time:

  my @s = qw/ foo bar /;
  for (@words) {
    for my $s (@s) {
      print if /$s/;
    }
  }
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You can get information about what perl is doing with your regexps using the -Dr switch if perl has been compiled for debugging, or with use re 'debug';. This produces a lot of scary-looking information about both compiling and running regular expressions, but if you ignore that and just look at the "Compiling ..." lines it will show you what is happening.

The last case above is where qr{} comes in: if we fill @s with qr{}'d expressions instead, we can avoid the recompile:

  my @s = map qr{$_}, qw/ foo bar /;
  for (@words) {
    for my $s (@s) {
      print if /$s/;
    }
  }
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Note that this only helps if you are using exactly the compiled regexp: if you interpolate it into something more, even if only to add an anchor, it will go back to recompiling each time.