package MyBase;

use strict;
use warnings;
use Exporter;

our @ISA = qw(Exporter);
our @EXPORT = qw(bar);

sub FOO() {42}

sub bar {
    return FOO();
}

1;

##</code><code>##

package MySub;


use strict;
use warnings;


sub _filter {
    my $module = $_[1];
    if ($module =~ m/^MyBase/) {
        foreach my $dir (@INC[1..$#INC]) {
            if (-e "$dir/MyBase.pm" && -f _) {
                open FH, '<', "$Bin/lib/MyBase.pm" or die $!;
                last;
            }
        }
        die "Couldn't find MyBase.pm in @INC\n" unless defined *FH;

        return
            \'',
            \*FH,
            sub {
                if (length $_) {
                    $_ =~ s/(sub FOO\(\)\s*\{)(\d+)(\})/${1}44${3}/;
                    return 1;
                }
                else {
                    return 0;
                }
            };
    }

    return ();
}

BEGIN {unshift @INC, \&_filter}
use MyBase;

use Exporter;
our @ISA = qw(Exporter);
our @EXPORT = qw(baz);


sub baz {
    return bar();
}

1;

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;
use FindBin qw($Bin);
use lib "$Bin/lib";

use MySub;

print baz(), "\n";