_       _     _
                             |     | |   | |
>perl -Mre=debug -e"for (qw( a a a a b b b a )) { qr/$_/ }" 2>&1 | find "Compiling"
Compiling REx "a"
Compiling REx "b"
Compiling REx "a"

##</code><code>##

for my $pat (...) {
   for my $string (...) {
      ... $string =~ /$pat/ ...
   }
}

##</code><code>##

for my $string (...) {
   for my $pat (...) {
      ... $string =~ /$pat/ ...
   }
}