Basicly a\n?b\n?c means match a followed by, perahps ? a newline \n followed by a b followed by, perahps ? a newline \n and a c

The m regex modifier (probably unneeded in my example) stands for multiline and the g one means globally ie all occurences are returned.

$count=()=$string=~/pattern/g idiom is used to count the occurences of pattern in $string infact $string=~/pattern/g with the g returns a list and the generic list () is provided and it's scalar value (ie the number of elements) is returned to the scalar $count

For shortness i put your example data into a doublequoted string using qq operator qq(abcdefab\ncdefa\nbcdef)

the rest is only print stuffs.

If you want to slurp a file into a string you can play with $/ aka input record separator, see perlvar and How do I read an entire file into a string?

L*