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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> ./test.pl foo.txt
foo.txt
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#!/usr/bin/perl -w

#
# input from command line
#
my ( $foo, $bar ) = @ARGV;
print "foo = $foo\n";
print "bar = $bar\n";
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#!/usr/bin/perl -w

#
# input from command line
#
my $foo = shift;
my $bar = shift;
print "foo = $foo\n";
print "bar = $bar\n";
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#!/usr/bin/perl
#
# embed perdoc somewhere (top or bottom) for script documentation.
#
use warnings;
use strict;
use Getopt::Long;
use Pod::Usage;

my $help = 0;
my $man  = 0;

GetOptions('help|?' => \$help, man => \$man ) or pod2usage(2);

pod2usage(-exitval => 0, -verbose => 1) if $help;
pod2usage(-exitval => 0, -verbose => 2) if $man;

my $file = shift;
pod2usage(-exitfile => 1, -verbose => 1) unless ( defined( $file ));
die("File $file is not a regular file.\n") unless ( -f $file );
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$#ARGV is the index of the last argument; @ARGV in scalar context is how many arguments.</small-but-crucial-distinction>

Not trying to be disingenuous, but it sounds to me like you need to take a step back and get yourself an overview of the basics. Try some tutorials right here on perlmonks. After that, when you come back with a question, you should first review one of the tutorials dealing with posting on perlmonks.