Any idea why it's not being called?

Because it's calling the tell() method on an IO::Handle object, not a IO::InnerFile object, and the reason it's croaking is that IO::Handle doesn't have a tell method. A solution is adding IO::Seekable to IO::Handle's inheritance

BEGIN {
    use IO::Seekable;
    push @IO::Handle::ISA, 'IO::Seekable';
}

use strict;
use warnings;

use Data::Dumper;
use IO::InnerFile;

open(my $fh, $0);

my $inner = IO::InnerFile->new($fh, '100', '50');

while (<$inner>) {
    print;
}

close($fh);

__output__

 Data::Dumper;
use IO::InnerFile;

open(my $fh, $0
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HTH

_________
broquaint