Your replacement has to be something tangible, not a pattern. However you can use the /e modifier to execute code in the replacement. Combining this with a capture group works like this:

#!/usr/bin/perl

use strict;
use warnings;

my $line = "perlmonk is a gREat site. i loVE it.\n";
print $line;
$line = ucfirst lc $line;
print $line;
$line =~ s/(\. [a-z])/uc($1)/eg;
print $line;
[download]

Giving this output:

perlmonk is a gREat site. i loVE it.
Perlmonk is a great site. i love it.
Perlmonk is a great site. I love it.
[download]

Be sure to have a good read of perlre as regexes and the s/// function are bedrocks of Perl.

HTH, Hippo