No need to sort if you just want the highest number:

my ($max_file, $max_num);
while (defined(my $file = <foo_*.bar>)) {
 ($max_file, $max_num) = ($1, $2)
  if   $file =~ /^(foo_(\d+)\.bar)$/
   and (!defined $max_num or $2 > $max_num);
}
print "$max_file\n";
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