my $filename; # used to remember the current file name
my $out = "output"; # prefix for output files
my $ofh; # output filehandle, currently closed
while (<>) {
    if ($ARGV ne $filename) {
        $filename = $ARGV;
        open $ofh, '>', "$out-$filename"
            or die "Can't write to '$out-$filename': $!\n";
    }
    print $ofh $_;
}