Dynamic Behavior

22. Symbolic references:

    $i = "foo"; $$i = 10; # Sets $foo to 10 ${"i"} = 10; # Sets $foo to 10 &$i(); # Calls foo(); push (@$i, 10, 20); # Pushes 10,20 into @foo
    

23. Run-time expression evaluation:

    while (defined($str = <STDIN>)) {
     eval ($str);
    print "Error: $@" if $@;
    }
    

    This is a tiny Perl shell, which reads standard input, treats $str as a small program, and puts the compilation and run-time errors in $@.

24. Dynamic substitutions. Use the /e flag for the s/// operator, to specify an expression instead of a pattern:

    $l = "You owe me 400+100 dollars"; $l =~ s/(\d+)\+(\d+)/$1 + $2/e;
    print $l; # prints "You own me 500 dollars"
    

25. Module and object method invocations (see also #20 and #21):

    $modulename->foo(); # Calls foo() in the module indicated by # $modulename