I'd have to agree with andyford. You code is doing the correct thing from a perl perspective:

#!/usr/bin/perl

use strict;
use warnings;

my $infile_name = shift( @ARGV );
print $infile_name, "\n";
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when called, does:

> ./test.pl foo.txt
foo.txt
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So are you expecting test.pl to be in the @ARGV array? This is one place where perl differs from other programming languages. @ARGV is not populated with the name of the invoked program (ala C) - that's in the magic variable $0.