my $argc = @ARGV;

if($argc != 1) {
 print "Usage: \$ $0 <scsfile>\n";
 exit;
}

my $scs = $ARGV[0];
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my $scs = shift or die "Usage: $0 <scsfile>\n";

Don't forget to check the success of your open calls.

You might want to escape the periods and anchor the ends of your regular expression. File::Basename might also come in handy, but may be severe overkill:

(my $css = $scs) =~ s/\.scs\z/.css/;

A heredoc would make the print statements somewhat more manageable. Roll on Perl 6 and intendable heredocs!