Your code doesn't compile, and it doesn't explain where $section comes from, which seems relevant since you intend for it to be the replacement portion of a substitution operator.

Here's an example of a substitution that does what you're asking for based on the example data you provided:

my @test_cases = (
    ['cat.1', 'cat(1)'],
    ['ld.so.conf.5', 'ld.so.conf(5)'],
);

foreach my $test (@test_cases) {
    my ($string, $want) = @$test;
    my $transformed = $string =~ s/\.(\d+)$/($1)/r;
    print "[$string] --> [$transformed] wanted: [$want]. Success: ", (
+$transformed eq $want ? "yes\n" : "no\n");
}
[download]

It's also an example of how to produce an SSCCE. Here's the output it produces:

[cat.1] --> [cat(1)] wanted: [cat(1)]. Success: yes
[ld.so.conf.5] --> [ld.so.conf(5)] wanted: [ld.so.conf(5)]. Success: y
+es
[download]