have you tried it like this

my $filepipe = "zcat $filename |";
my $bw = File::ReadBackwards->new($filepipe) or die "can't read $filep
+ipe: $!\n";
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but i'm not sure about that...

but anyway... the module probably has to have read in the complete file into memory or temporary store it somewhere completly, so it will probably simpler to decompress it in the first place and than reading it backwards.