grep expr, list
grep {block} listEvaluates expr or block in a Boolean context
for each element of list,
temporarily setting $_ to each element in turn.
In list context, it returns a list of those elements
for which the expression is true.
Mostly used like Unix grep where expr is a search
pattern, and list elements that match are returned.
In scalar context, grep returns the number of times the
expression was true.
For example, presuming @all_lines contains lines of code, this example weeds out
comment lines:
@code_lines = grep !/^#/, @all_lines;