use re 'debug';

for (1..2) {
   $re_str = $_;
   print("***** $_ *****\n");
   /$re_str/;
}

$re_str = "\\d";
for (3..4) {
   print("***** $_ *****\n");
   /$re_str/;
}

##</code><code>##

***** 1 *****
Compiling REx `1'
Guessing start of match, REx "1" against "1"...

***** 2 *****
Compiling REx `2'
Guessing start of match, REx "2" against "2"...

***** 3 *****
Compiling REx `\d'
Matching REx "\d" against "3"

***** 4 *****
Matching REx "\d" against "4"     <--- No "Compiling" before this.