Another WTDI.

A one liner (I'm no true obfuscator):
perl -ne 'undef@_;while($_){$_[0]=($_&1&&1||0).$_[0];$_>>=1}print"@_\n"'

Or a more sensible sub version with the same logic.

sub to_bin {
  my $num = shift;
  my $ret = '';
  while ($num) {
    $ret = (($num % 2) ? 0 : 1) . $ret;
    $num >>= 1;
  }
  return $ret;
}
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