diff --git a/lib/Minilisp.pm b/lib/Minilisp.pm
index 8fa1294..63db6a6 100644
--- a/lib/Minilisp.pm
+++ b/lib/Minilisp.pm
@@ -34,26 +34,42 @@ sub tokenize {
   $str =~ s/^\s+//;
   $str =~ s/\s+$//;
 
+  my $debug = sub {};
+
+  $debug = sub {
+    my $msg = shift;
+    my $strcpy = $str;
+    $strcpy =~ s/^/  /mg;
+    print "-- $msg\n$strcpy\n";
+  } if !!$ENV{LEXER_DEBUG};
+
+  $debug->("Begin parsing");
+
   while ($str)
     {
       if ($str =~ s/^;.*\n//)
         {
+          $debug->("Comment");
           # Comment. do nothing
         }
       elsif ($str =~ s/^\(//)
         {
+          $debug->("Left parenthesis");
           push @tokens, { type => LPAREN };
         }
       elsif($str =~ s/^\)//)
         {
+          $debug->("Right parenthesis");
           push @tokens, { type => RPAREN };
         }
       elsif($str =~ s/^'\(//) # short notation for lists
         {
+          $debug->("Short list");
           push @tokens, { type => LIST }, { type => LPAREN };
         }
       elsif($str =~ s/^'([^\s()"]+)//)
         {
+          $debug->("Keyword $1");
           push @tokens, {
             type => KEYWORD,
             value => $1,
@@ -61,6 +77,7 @@ sub tokenize {
         }
       elsif($str =~ s/^"(([^"\\]|\\.)+)"//)
         {
+          $debug->("String '$1'");
           my $value = $1;
 
           my %special_chars = (
@@ -86,6 +103,7 @@ sub tokenize {
             }
           elsif($ident =~ /^-?([0-9]+|[0-9]*\.[0-9]*)$/)
             {
+              $debug->("Number '$1'");
               if($ident =~ s/^-//)
                 {
                   $ident = 0 - $ident;
@@ -98,6 +116,7 @@ sub tokenize {
             }
           else
             {
+              $debug->("Identifier '$1'");
               push @tokens, {
                 type => IDENT,
                 value => $ident,