You need to parse the command line options (see Getopt::Long) and then run the functions based on that. For instance:

use strict;
use Getopt::Long;

my $a_opt;

my $options = GetOptions('a' => $a_opt);

if($a_opt) { 
  Foo1();
  Foo2();
  Foo3();
}

# Rest of program here
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