Passing a value as an argument (@ARGV) is not the same as reading from stdin, try:

#!/usr/bin/perl 
use warnings;
use strict;

my $value;
if (defined $ARGV[0]) {
    $value = $ARGV[0];
}
else {
    $value = <STDIN>;
}

print "<$value>\n";
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Run that using your two methods, and note the difference. Also lookup chomp.
I hope that helps.