sub format_time{
  
  my ( $h, $m, $s ) = @_;

  ...

  if ( length( $h ) < 2 ) {
    $h = "0" . $h; 
  }
  if ( length( $m ) < 2 ) {
    $m = "0" . $m; 
  }
    
  return $h . $m . $s2;
}

print format_time( 9, 5, 13 )."\n";
print format_time( 12, 34, 50 )."\n";

__OUTPUT__
 905
1235