The substitution operator returns the number of substitutions in scalar context, so you could write it like this:

use strict;
use warnings;

my @strings = qw/ one two three /;

my $counter = 0;
for ( @strings ) {
  $counter += s/o/O/; # only add one if s/// is successful
}
print "$counter replacements\n";
print "@strings\n";

$counter = 0;
for ( @strings ) {
  $counter += s/e/E/g; # add number of replacements
}
print "$counter replacements\n";
print "@strings\n";
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UPDATE: When you want to use the counter in your replacement string, then adding one should do the trick, not forgetting to remove it later:

use strict;
use warnings;

my @strings = qw/ one two three four /;

my $counter = 1;
for ( @strings ) {
  $counter += s/o/O$counter/; # only add one if s/// is successful
}
$counter--;
print "$counter replacements\n";
print "@strings\n";
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