my @str= qw(ssg.nal ssg_00.nsr ssg_00.nsn ssg_00.nss  xxx_01.txt xxx_03.dat);

my %starts;

foreach (@str) {
#increment the value for the hash element that has the
#prefix as a key.
	$starts{$1}++ if /^([a-zA-Z0-9]+)/;
}

# print all the prefixes and number of files associated
foreach (keys %starts) {
	print "$_ = $starts{$_}\n";
}