D.5.1 grep and map

The grep function selects elements from its argument list, based upon the result of an expression that's repeatedly evaluated for its truth value, with the $_ variable successively set to each element in the list. For example:

@bigpowers = grep $_ > 6, 1, 2, 4, 8, 16; # gets (8, 16)
@b_names = grep /^b/, qw(fred barney betty wilma);
@textfiles = grep -T, <*>;

The map operator is similar, but instead of selecting or rejecting items, it merely collects the results of the expression (evaluated in a list context):

@more = map $_ + 3, 3, 5, 7; # gets 6, 8, 10
@squares = map $_ * $_, 1..10; # first 10 squares
@that = map "$_\n", @this; # like "unchop"
@triangle = map 1..$_, 1..5; # 1,1,2,1,2,3,1,2,3,4,1,2,3,4,5
%sizes = map { $_, -s } <*>; # hash of files and sizes

D.5.2 The eval Operator (and s///e)

Yes, you can construct a piece of code at runtime and then eval it. This process forces a dynamic compilation of the code inside the eval. This compilation is actually rather useful, because you can get some compile-time optimizations (like a compiled regular expression) at runtime. You can also use it to trap otherwise fatal errors in a section of code: a fatal error inside the eval merely exits the eval and gives you an error status.

For example, here's a program that reads a line of Perl code from the user and then executes it as if it were part of the Perl program:

print "code line: ";
chop($code = <STDIN>);
eval $code; die "eval: $@" if $@;

You can put Perl code inside the replacement string of a substitute operator with the e flag. This is handy if you want to construct something complicated for the replacement string, such as calling a subroutine that returns the results of a database lookup. Here's a loop that increments the value of the first column of a series of lines:

while (<>) {
  s/^(\S+)/$1+1/e; # $1+1 is Perl code, not a string
  print;
}

Another use of eval is as an error-trapping mechanism:

eval {
  &some_hairy_routine_that_might_die(@args);
};
if ($@) {
  print "oops... some_hairy died with $@";
}

Here, $@ will be empty as long as the eval block worked; otherwise, it will have the text of the die message.