#!/usr/bin/perl -
                ^
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I'm not sure you want the dash at the end of the shebang line...

It doesn't do any harm when invoking the code like you did, but when I try to run your sample code/data without explicitly calling perl (rather letting the OS handle the shebang), I'm getting

$ cat file.txt | ./664708.pl
Can't locate object method "a" via package "b" (perhaps you forgot to 
+load "b"?) at - line 1.
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I suppose this is because what comes in through stdin is being interpreted as Perl code, rather than as content to be read via <> ...