my $posn = -1;
my $same = 1;
while(defined $same and $posn <= length $files[0]){
 my $chr = substr($files[0], ++$posn, 1);
 for my $name (@files) {  
   undef $same and last if(substr($name, $posn, 1) ne $chr
                                 or length $name < $posn);
 }
}

print 'Prefix is "', substr($files[0], 0, $posn), '"';