You don't want a switch statement. What switch does is take a variable and do different stuff based on the value held within that variable. You need to set $_ to something (say within a loop) otherwise the results will be unexpected. You have several key value pairs in your %opt hash and you just want to do stuff based on what they hold. I expect this will do what you want. Pick whichever variations of syntax/function that work best for you.

defined $opt{1} and read_structure_type1($opt{1});
defined $opt{2} and read_structure_type2($opt{2});
read_structure_type3($opt{3}) if defined $opt{3};

defined $opt{f} and do{ $fldname = $opt{f} };            
$length   = $opt{l} if defined $opt{l};
$location = defined $opt{L} ? $opt{L} : 'default';
$infile   = $opt{i} || 'default';
$dirname  = $opt{d} || '/dev/null';

do {usage(); exit; } if defined $opt{h} or not %opt;
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