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";