I was asked

How do we know that perl is smart enough not to recompile the regexp with every iteration of the inner loop nonetheless?

We can use use re 'debug'; to see the difference.

use re 'debug';

for my $i (1..2) {
   for my $re (qw( abc def )) {
      $i =~ /$re/;
   }
}
[download]

ouputs

Compiling REx `abc'         <------------
size 3 first at 1
   1: EXACT <abc>(3)
   3: END(0)
anchored `abc' at 0 (checking anchored isall) minlen 3 
Freeing REx: `abc'
Compiling REx `def'         <------------
size 3 first at 1
   1: EXACT <def>(3)
   3: END(0)
anchored `def' at 0 (checking anchored isall) minlen 3 
Freeing REx: `def'
Compiling REx `abc'         <------------
size 3 first at 1
   1: EXACT <abc>(3)
   3: END(0)
anchored `abc' at 0 (checking anchored isall) minlen 3 
Freeing REx: `abc'
Compiling REx `def'         <------------
size 3 first at 1
   1: EXACT <def>(3)
   3: END(0)
anchored `def' at 0 (checking anchored isall) minlen 3 
Freeing REx: `def'
[download]

while

use re 'debug';

for my $re (qw( abc def )) {
   for my $i (1..2) {
      $i =~ /$re/;
   }
}
[download]

outputs

Compiling REx `abc'         <------------
size 3 first at 1
   1: EXACT <abc>(3)
   3: END(0)
anchored `abc' at 0 (checking anchored isall) minlen 3 
Freeing REx: `abc'
Compiling REx `def'         <------------
size 3 first at 1
   1: EXACT <def>(3)
   3: END(0)
anchored `def' at 0 (checking anchored isall) minlen 3 
Freeing REx: `def'
[download]

If the interpolated variables are still the same as they were the last time the regexp was compiled, the regexp is not recompiled.

Update: We get the same results with /\b$re\b/.