my $hyphens = 0;
    $hyphens++ while $str =~ /-/g;

##</code><code>##

    my $hyphens = length $1 if $str =~ m{\A(-*)};

##</code><code>##

$ perl -Mstrict -Mwarnings -le '
> my @strings = qw{--aacgtACG ctgGTTAtga};
> foreach my $str ( @strings )
> {
>     my $hyphens = length $1 if $str =~ m{\A(-*)};
>     print qq{$str - $hyphens};
> }'
--aacgtACG - 2
ctgGTTAtga - 0
$