You're ignoring the result of the recursive call.

&spl($total) if ($len != 1);
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should be

$total = &spl($total) if ($len != 1);
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Now, what we have here what is called tail-end recursion. The recursion is the last thing in the function. That means you can use a loop instead.

sub spl {
   my ($num1) = @_;    
   while (1) {
      my $total = 0;
      $total = $total + $_ for (split '', $num1);
      my $len = length ($total);
      if ($len == 1) {
         return $total;
      }
      $num1 = $total;
   }
}
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After some cleaning up, we get:

sub spl {
   my ($n) = @_;    
   while (length($n) > 1) {
      my $total = 0;
      $total = $total + $_ for split '', $n;
      $n = $total;
   }
   return $n;
}
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or even

use List::Util qw( sum );

sub spl {
   my ($n) = @_;    
   while (length($n) > 1) {
      $n = sum split '', $n;
   }
   return $n;
}
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Update: Added last snippet.