I really like generating code on the fly. It is fairly easy if you watch your back(slashes):

• generate a string with the code for a subroutine (that's the fun part)
• generate an anonymous subroutine from it $sub= eval "sub { $sub_in_a_string }";
• use the subroutine $sub->( @args);

et voila!

Here is a stupid example:

#!/bin/perl -w
use strict;

# very simple rules: s="<string>" or v=<number>
my %rules=( 's="toto"' => sub { print "toto!\n";     },
            'v=2'      => sub { print "waouh! 2!\n"; },
          );
# values to test
my @pairs=( toto => 2, toto => 1,  tata => 2,  tutu => 1);

my @rules= make_rules( %rules);                         # create the r
+ules

while (@pairs)
  { my $string= shift @pairs;
    my $value= shift @pairs;
   check_rules( $string, $value);                       # check all ru
+les
   print "\n"; 
}

sub check_rules 
  { my( $string, $value)= @_;
    foreach my $rule (@rules)
      { my( $you_like, $do_it)= @{$rule};
        $do_it->() if( $you_like->( $string, $value));  # run routine 
+if rule applies
      }
  }

sub make_rules
  { my %rules= @_;
    my @rules;
    foreach my $exp (keys %rules)
      { my $sub_str= ' my( $s, $v)= @_;' ;               # get the arg
+uments
        if( $exp=~ m/^s=(".*?")$/)                       # s="<string>
+"
          { $sub_str .= "return 1 if( \$s eq $1);" ; }   # watch for t
+he \, 
                                                         #    $s is a 
+variable in the sub
                                                         #     while $
+1 is a variable in make_rules and a constant in the sub
        elsif( $exp=~ m/^v=(\d+)$/)                      # v=<number>
          { $sub_str .= "return 1 if( \$v == $1);" ; }   # watch for t
+he \
        else 
          { die "syntax error in rule $exp"; }
        my $sub= eval "sub { $sub_str }";                # create the 
+anonymous sub
        push @rules, [$sub, $rules{$exp}];               # store it wi
+th the sub to run
      }
    return @rules;
  }
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You can see an example in Ugly XML processing looking for a pure XML solution: I create a hairy regexp in make_wrapper, which is used in wrap -- calling the wrapper in wrap should really be written $wrapper{$tag}->( @_);