I'm not even sure what you're trying to achieve here.

• Call str with one argument, but assign @_ to two arguments, thus rendering the second one undef
• Then you assign to the second var anyway. Fine, I guess, but I would have done something like

  sub str {
    my ($want_num) = @_;
    my $width = '000';
    # ...
  [download]

• Then you replace periods in the string representation of 123.456 (which could be "123,456" or even "١٢٣٫٤٥٦" by this time) with commas, resulting in not much.
• And then print whatever the replaced result is (let’s say "123,456") concatenated with "000" (why is that even called ‘width’?), resulting in an output of 123,456000.
• Then you put the return value of that print in $want_num for not much reason, as print returns true on a successful print, and the value doesn’t escape the subroutine anyway.
• str and thus the whole program then returns true after printing the above value.

Pray tell, what did you want to achieve?