foreach $file (@ARGV) {
    process($file, 'fh00');
}

sub process {
    my($filename, $input) = @_;
    $input++;               # this is a string increment
    unless (open($input, $filename)) {
      print STDERR "Can't open $filename: $!\n";
      return;
    }

    local $_;
    while (<$input>) {              # note use of indirection
       if (/^#include "(.*)"/) {
           process($1, $input);
           next;
       }
       #...                # whatever
    }
}