I decided to go with using fork to invoke the reloadable code. In my final implementation the child will use require to load a bunch of code that the parent doesn't need so that when the child process terminates, that code gets unloaded. I wanted a simple way for the child to send a message on its deathbed to the parent. I considered various IPC options, but in the end decided it would be simplest to just write a message to a file. The sample code below includes verbose messaging to easily follow what's happening:

./sigsuccess.pl
test> sig
INT: sigInt
test> loop
^C
child 38021: INT
child 38021: Reloading sigsuccess.pl
parent 38020: ignore INT
parent 38020: CHLD (reload)
parent 38020: Reloading...
test> loop
^C
child 38023: INT
child 38023: Reloading sigsuccess.pl
parent 38020: ignore INT
parent 38020: CHLD (reload)
parent 38020: Reloading...
test> exit
child 38024: Child exiting
parent 38020: CHLD ()
parent 38020: exiting after waitpid

use B;
use Term::ReadLine;

$SIG{INT} = sub { print "parent $$: ignore INT\n" };
my $comm = "message.$$";
reload:
if ($pid = fork())
{
  my $reload = 0;
  local $SIG{CHLD} = sub
    {
      my $request = "";
      if (open(my $fh,$comm))
      {
        $request = <$fh>;
        chomp $request;
        close $fh;
        unlink $comm;
        $reload = 1 if $request =~ m/^reload$/;
      }
      print "parent $$: CHLD ($request)\n";
    };
  waitpid($pid,0);
  if ($reload)
  {
    print "parent $$: Reloading...\n";
    goto reload;
    }
  print "parent $$: exiting after waitpid\n";
} else {
  $SIG{INT} = \&sigInt;
  $term = Term::ReadLine->new("sigsuccess");
  while ($resp = $term->readline("test> "))
  {
    if ($resp =~ m/^exit/) { last; }
    elsif ($resp =~ m/^loop/) { loop: goto loop; }
    elsif ($resp =~ m/^reload/) { reload(); last; }
    elsif ($resp =~ m/sig/) { sig() }
    else { print "What?\n"; }
  }
  print "child $$: Child exiting\n";
  exit;
}

sub codeToName
{
  my ($coderef) = @_;
  my $cv = B::svref_2object($coderef);
  return "" unless defined($cv) && defined($cv->GV);
  return $cv->GV->NAME;
}
sub reload
{
  print "child $$: Reloading sigsuccess.pl\n";
  open my $fh, ">$comm";
  print $fh "reload\n";
  close $fh;
  exit(0);
}
sub sigInt { print "\nchild $$: INT\n"; reload() }
sub sig
{
  map
  {
    print "$_: ",codeToName($SIG{$_}),"\n"
      if defined($SIG{$_}) && ref($SIG{$_}) eq "CODE"
  } sort { $a cmp $b } keys %SIG;
}
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