You can't use subscripts in a variable used as a filehandle. This applies to the diamond operator as well as print statements. I.e. print $_[0] "foo"; won't work either.

Update: Here's the relevant bit from perldoc perlop:

     If what's within the angle brackets is neither a filehandle
     nor a simple scalar variable containing a filehandle name,
     typeglob, or typeglob reference, it is interpreted as a
     filename pattern to be globbed, and either a list of
     filenames or the next filename in the list is returned,
     depending on context.  This distinction is determined on
     syntactic grounds alone.  That means "<$x>" is always a
     readline() from an indirect handle, but "<$hash{key}>" is
     always a glob().  That's because $x is a simple scalar
     variable, but "$hash{key}" is not--it's a hash element.
[download]

By the way, please test your example code before pasting it. Yours had several errors unrelated to the issue you are seeing.

-sauoq
"My two cents aren't worth a dime.";