I also think that Perl has an internal cache that avoids recompiling the same regex all over again.

The ops that compile regex patterns (qr//, m// and s///) remember the last pattern they compiled.

                            _       _     _
                             |     | |   | |
>perl -Mre=debug -e"for (qw( a a a a b b b a )) { qr/$_/ }" 2>&1 | fin
+d "Compiling"
Compiling REx "a"
Compiling REx "b"
Compiling REx "a"
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So if you compile the same pattern many times in a row, you'll get savings.

It's useful for code that looks like

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

But not for code that looks like

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