open my $fds[$_], '<', "datafile$_" or die $!;
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You can't use my on an array element.

    my $line = <$fds[$fdn]>;
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From I/O Operators:

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 $hash{key} is not--it's a hash element. Even "<$x >" (note the extra space) is treated as "glob("$x ")", not "readline($x)".