#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

sub foo {
    return "foo:\t" . $_[0];
}

sub contract_on {
  my $func = shift;
  
  my $sub = sub {
      my $i = shift;
      warn "need an integer!" unless ($i =~ /\d+/);
      return $func->($i);
  };
  
  no strict 'refs';
  no warnings;
  *{'foo'} = $sub; # woher bekommt man den Namen der Funktion?
}


print foo('X') . "\n";
contract_on \&foo;
print foo('X') . "\n";