#!/usr/bin/perl -w

use strict;

sub on {
  my($f1,$f2,$l)=@_;

  my $parmtype = ref $f1;
  print "f1: $parmtype\n";

  if ($parmtype eq 'CODE') {
    local $_=$$l;
    if (&$f1) {
      local *_=$l;
      &$f2($l);
    }
  };
}

my $foo="abba is foo";
on sub{ /foo/ }, sub { tr/ab/ba/; }, \$foo;
print "$foo\n";
on "bla", sub { tr/ab/ba/; }, \$foo;