package MyPack;
use strict;

sub new($$)
{
    my $invocant = shift;
    my $class = ref($invocant) || $invocant;
    my $self = { };
    bless($self, $class);
    return $self;
}

sub foo($)
{
    my $self = shift;
    print "foo is unmodified.\n";
}

sub changeFoo($)
{
    my $line;
    my $code;
    while ($line = <DATA>) {
        $code .= $line;
    }
    eval $code;
}

1;

__DATA__

sub foo($) 
{
    my $self = shift; 
    print "foo has been modified.\n";
}