As a followup, having tested the "do" option, I found that using "require" was altogether the best option, especially in terms of memory usage on large files.

   $client = $Options{'c'};
   #
   # call in the relevant client code
   #
   require "$client.segment";
   #
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and the $client.segment file contains

sub DoClient
        {
        # client 'A'
        my $EURO       = 1.35; 
        if ( m/(\S+),(\S+),(\S+),(\S+),(\S+),(\S+)/o )
                {
                @fields        = split(/,/);
                ......
                }
        }1;
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