Hi,
I don't know if this classifies as more elegant than your solution ..

use strict;
use warnings;

my @lines = ( "??????????????", "123" );
my $conv;
for my $line ( @lines ){
  $conv="";
  my @c = split('', $line);
  $conv .= ($c[$_] || 0) for 0..5;
  print "$conv\n";
}
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si_lence