I don't know how complex your program would be, but I usually just use,

$ cat file.txt
a
b
c

$ cat stdin.pl
#!/usr/bin/perl -l
while (<>) {
 $_ = uc;
 print;
}

$ perl stdin.pl file.txt
A
B
C

$ cat file.txt | perl stdin.pl
A
B
C
[download]

Or, just perl -lpe '$_=uc' if it's really that simple. Anyway, I don't see anything wrong with your code.

Update: Fixed missing -l (only the dash was there). Thanks almut :-)