## When we are looking on REGEXP for something that is after other,
## genreally "\n", we forget about the beggin of the string,
## that enables an occurrence that is not after "\n".
## Soo, to do this in only one REGEXP we use (?:^|\n)

  ## remove comments:
  $str =~ s/(?:^|\n)[ \t]*#//gs
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Seems that you "forgot" or "don't know" something as well, namely the value of /m. Furthermore, it seems you want a .* after the # as well, but not in combination with /s!

I'd write the regex as:

  $str =~ s/^[ \t]*#.*\n//gm;
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Abigail