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my $opt = 'ah1:2:3:f:l:L:i:d:';
my ($isascii, $fmtfile, $fldname, $length, $location, $infile, $dirnam
+e);
...
        case {$opt{d}} { $dirname = $opt{d}; next; }
        case {$opt{h}} { usage(); exit;            }
}

switch ($_) {
        case {$opt{1}} { read_structure_type1($opt{1}); next; }
...
        case {$opt{d}} { $dirname = $opt{d};            next; }
        else {usage();                       exit; }
}