open (DAT, "@ARGV");
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"@ARGV" is short for join( $", @ARGV ).    If you just want the first argument from the command line then use $ARGV[0] instead.    You should really be using the three argument form of open and you should always verify that the file opened correctly, so:

        open mt $DAT, '<', $ARGV[ 0 ] or die "Cannot open '$ARGV[0]' $
+!";
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        while ($line = <DAT>){
                do (@word = split (/\W/, $line));
                        foreach $word (keys %charCount){
                                do (@letter = split (/\w+/, $word);
                                        $letter = (keys %charcount)}
                if ($charCount){$char}){
                        $charCount{$char}++;
                }else {
                        $charCount{$char}=1;
                }
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Since you say that you only want letters you need something like this:

        my %charCount;
        while ( my $line = <$DAT> ) {
                my @letters = $line =~ /[a-zA-Z]/g;
                foreach my $char ( @letters ) {
                        $charCount{ $char }++;
                }
        }

        foreach my $char ( keys %charCount ) {
                print "$char => $charCount{$char}\n";
        }
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        close(DAT, "@ARGV");
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close only accepts one argument, the filehandle that was previously opened.

        close $DAT;
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