Update, since this was the first in a search for "perl cgi param multiple values."

Newer versions of CGI.pm will generate a warning if you use

my @vals = $q->param('input');
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Better to use

my @vals = $q->multi_param('input');
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From man CGI: "Warning - calling param() in list context can lead to vulnerabilities if you do not sanitise user input as it is possible to inject other param keys and values into your code. This is why the multi_param() method exists, to make it clear that a list is being returned, note that param() can still be called in list context and will return a list for back compatibility."

And "If you call param() in list context with an argument a warning will be raised by CGI.pm, you can disable this warning by setting $CGI::LIST_CONTEXT_WARN to 0 or by using the multi_param() method instead"