Simple but subtle = @ARGV != STDIN. The difference is that arguments are fed to the program at initialization time = they make up the values of the automatic @ARGV array - while STDIN is what is fed into a program *after* initialization via pipes, user input, or other redirects. So, if you want to pipe output to a perl app, use the <STDIN> filehandle to capture it, like so:

#!/usr/bin/perl -w
my $line = <STDIN>;
print "Line is: " . $line;

$ echo 'echo' | ./teststdin.pl 
Line is: echo
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This also works for redirecting files:

$ cat echo.txt 
echoed file
$ ./teststdin.pl < echo.txt
Line is: echoed file
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Hope this helps!