Several updates, documentation, and fixes for challenge 2

For example, I can run exercise one through this script:

> perl6 ch-2.p6 ch-1.p6
(0 0)
(0 0)
(0 0)
Expected error, because the input doesn't represent a line.
Variables history:
disc = (8, 0)

1 files changed, 43 insertions, 24 deletions

diff --git a/challenge-027/noud/perl6/ch-2.p6 b/challenge-027/noud/perl6/ch-2.p6
index 0d7e6d9f8f..9b6b316e31 100644
--- a/challenge-027/noud/perl6/ch-2.p6
+++ b/challenge-027/noud/perl6/ch-2.p6
@@ -5,37 +5,56 @@
 #
 # After the above operations, it should list $x historical value in order.
 
+# The idea behind this solution is to first extract all defined variables
+# from the original script. After that we create a hash %var_hash_ that
+# updates the current values of each of the defined variables after each
+# semicolon. The new script is executed using the dangerous EVAL method.
+#
+# Usage:
+#   perl6 ch-2.p6 filename-of-script.p6
+
 # I hope I don't get banned from the Perl Weekly Challenge club for using EVAL
 # in this problem. ;)
 use MONKEY-SEE-NO-EVAL;
 
-my @variables = ();
-my $pattern = /my\s*\$(\w+)/;
-
-my $exec_prog = "";
-for 'exm.p6'.IO.slurp.split(";") -> $line {
-    my @line_var = ($line ~~ $pattern).values;
-    if (@line_var.elems > 0) {
-        @variables = (|(@line_var), |(@variables));
+sub MAIN(Str $filename) {
+    # Collect all variables in program.
+    my @variables = ();
+    for $filename.IO.slurp.split(";") -> $line {
+        my @line_var = ($line ~~ /my\s*\$(\w+)/).values;
+        if (@line_var.elems > 0) {
+            @variables = (|(@line_var), |(@variables));
+        }
     }
-    $exec_prog = "$exec_prog $line\;";
-    # After every line update %var_hash_ with the current variable values.
-    for @variables -> $x {
-        $exec_prog = "$exec_prog \%var_hash_\.push: ($x => \$$x)\;";
+
+    my $exec_prog = "";
+    for $filename.IO.slurp.split(";") -> $line {
+        $exec_prog = "$exec_prog $line\;";
+        # After every line update %var_hash_ with the current variable values.
+        for @variables -> $x {
+            $exec_prog = "$exec_prog
+                if (not DYNAMIC::<\$$x> === Nil) \{
+                    \%var_hash_\.push: ($x => DYNAMIC::<\$$x>)\; \}\;";
+        }
     }
-}
 
-my %var_hash_;
-EVAL $exec_prog;  # https://xkcd.com/292/
-for %var_hash_.kv -> $var, @hist {
-    print("$var = (");
-    my $last = @hist[0];
-    print("$last");
-    for @hist[1..*] -> $next {
-        if ($last != $next) {
-            print(", $next");
-            $last = $next;
+    my %var_hash_;
+    EVAL $exec_prog;  # https://xkcd.com/292/
+
+    say "Variables history:";
+    for %var_hash_.kv -> $var, @hist {
+        my @hist_ = @hist.grep({ not $_.^name === "Any" });
+        if (@hist_.elems > 0) {
+            print("$var = (");
+            my $last = @hist_[0];
+            print("$last");
+            for @hist_[1..*] -> $next {
+                if ($last != $next) {
+                    print(", $next");
+                    $last = $next;
+                }
+            }
+            print(")\n");
         }
     }
-    print(")\n");
 }