use warnings;
use strict;

use File::Basename qw(fileparse);

#Beispielnamen
my @files = qw(123.txt foo123456789.txt bar123456789.tst.txt);

for my $filename (@files) {
    # Dateinamen zerlegen (an der letzten Extension)
    my ($name, $dir, $ext) = fileparse($filename, qr/\.[^.]*/);
    if (length($name) > 8) {
        $name = substr($name, 0, 7) . '~';
    }
    print "$filename => $name$ext\n";
}