Or something slightly different yet the same

while ( <FILE> ) {
   $count += $_ =~ s/foo/foo/g;
   print "$count so far\n";
}
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If you do $count = $_ =~ s/foo/foo/g; $count will have how many replacements happened on that line. Instead of using a temp variable to hold it, I simply += it as we go.. I should probably benchmark it to see how it compares, but its late.

/* And the Creator, against his better judgement, wrote man.c */