that for example "sigma" matches "sig<SOMETHING>ma"?

why, of course. But that depends on what you mean by <SOMETHING> which is a period in regexp speak, followed optionally by a quantifier (see perlre):

$text =~ /sig.*?ma/; # matches sigma and sigima 
                     # (zero or more somethings, non-greedy match)
$text =~ /sig.+?ma/; # matches sigoma and sigilima
                     # (1 or more somethings, non-greedy match)
$text =~ /sig.?ma/;  # matches sigma and sigima, but not sigilima
                     # (zero or 1 something)
$text =~ /sig.ma/;   # just one something
[download]

You probably know all of this, but that would be the answer to your question as written, without assuming that there's more to it than what you have expressed.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}