my $x = "<0>";

for (('<1>','<2>','<3>')) {
   $x =~ s/(<0>)/${1}${_}/;
}

##</code><code>##

<0>
<3>
<2>
<1>

##</code><code>##

<0>
<1>
<2>
<3>

##</code><code>##

my $x = "<0>";

for (('<1>','<2>','<3>')) {
   if ($x =~ /<\d>/gc) {
      $x =~ s/(\G<\d>)/${1}${_}/g;
   }
}