my $sClass = derive_class($foo);
my $oWhatWasOnceASwitchStatementCase = $sClass->new();
$oWhatWasOnceASwitchStatementCase->frobnicate();

##</code><code>##

use strict;
use warnings;

#pretend command line is indirect user input
my $foo = \@ARGV;  #indirect user input

#this replaces your switch statement
# use derive_class($foo) to select the class
# use frobinate() to replace the action for each if statement branch

my $sClass = derive_class($foo);
my $oWhatWasOnceASwitchStatementCase = $sClass->new();
$oWhatWasOnceASwitchStatementCase->frobnicate();

#dummy implementation of derive_class($foo)
#converts command line arguments to a class
sub derive_class {
  my ($sClass) = $foo->[0];
  $sClass = 'Demo::Base' unless $sClass =~ /^Demo::[AB]$/;
  return $sClass;
}

#here are your class definitions
#implement default case
{
  package Demo::Base;
  sub new {
    my ($sClass) = @_;
    my $self = {};
    return bless($self, $sClass);
  }
  sub frobnicate {
    my ($self) = @_;
    #do similar_to_a
    print "I'm defaulting to something similar to A\n";
  }
}

#implement when A
{
  package Demo::A;
  use base 'Demo::Base';

  sub frobnicate {
    my ($self) = @_;
    #do A
    print "I'm doing A\n";
  }
}

#implement when B
{
  package Demo::B;
  use base 'Demo::Base';

  sub frobnicate {
    my ($self) = @_;
    #do B
    print "I'm doing B\n";
  }
}