I have interpreted the question as follows: Given a set of file names in an array, find the number of files that share the same alphanumeric prefix so that I know how many files I will be deling with when I use $prefix* as a glob.

I have added to the data to illustrate the point.

my @str= qw(ssg.nal ssg_00.nsr ssg_00.nsn ssg_00.nss  xxx_01.txt xxx_0
+3.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";
}
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gives

ssg = 4
xxx = 2
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Let me know if this is what you had in mind.