#!/usr/bin/perl -w
use strict;
use IO::File;

my $plain = IO::File->new('WEEK3.csv', O_RDONLY)
  or die "cannot open plain :: $!";

my $zipped = IO::File->new('unzip -qq -p WEEK3.zip |')
  or die "cannot open zipped :: $!";

print "plain IO::File\n"     if $plain->isa('IO::File');
print "plain IO::Handle\n"   if $plain->isa('IO::Handle');
print "plain IO::Seekable\n" if $plain->isa('IO::Seekable');
print "plain can seek\n"     if $plain->can('seek');

print "zipped IO::File\n"     if $zipped->isa('IO::File');
print "zipped IO::Handle\n"   if $zipped->isa('IO::Handle');
print "zipped IO::Seekable\n" if $zipped->isa('IO::Seekable');
print "zipped can seek\n"     if $zipped->can('seek');

use Fcntl qw(SEEK_END);

if ($zipped->isa('IO::Seekable')) {
    print "seeking\n";
    $! = 0;
    my $rc = $zipped->seek(0, SEEK_END);
    print "rc == $rc :: $!\n";
}

$! = 0;
if ($plain->isa('IO::Seekable')) {
    print "seeking\n";
    <$plain>;
    my $rc = $plain->seek(0, SEEK_END);
    print "rc == $rc :: $!\n";
}
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Output

plain IO::File
plain IO::Handle
plain IO::Seekable
plain can seek
zipped IO::File
zipped IO::Handle
zipped IO::Seekable
zipped can seek
seeking
rc ==  :: Illegal seek
seeking
rc == 1 ::
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