@strings = qw(abc-def abc--def abc-d-ef abcdef ---abc--def--);
for my $string (@strings) {
  print "<$string> " . ($string =~ tr/-// == 1 ? "ok" : "not ok");
}
=>
<abc-def> ok
<abc--def> not ok
<abc-d-ef> not ok
<abcdef> not ok
<---abc--def--> not ok