I suspect the problem is in the manner of sorting. If you did a string sort on the array of names, foo_10.bar would come before foo_2.bar. A numeric sort on captured digits should work.

Here is a stab at it:

# needs several foo_<n>.bar
{
    my $re = qr/foo_(\d+)\.bar$/;
    sub seq_num {
        $_ = shift;
        m/$re/;
        return $1;
    }
}
my @files = </dir/to/use/foo_*.bar>;
my @sorted_files = sort { seq_num($b) <=> seq_num($a) } @files;
my $next = 1 + seq_num($sorted_files[0]);
open NEXT, "> /dir/to/use/foo_$next.bar";
# print content to NEXT
close(NEXT);
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A full solution would special-case @files for 0 or 1 elements.

After Compline,
Zaxo