What's wrong with if's? Personally I think they make your program much easier to read and understand...

#!/usr/local/bin/perl -w
use Getopt::Long;
use Pod::Usage;
use strict;

# Parse command line arguments and assign corresponding variables
GetOptions
(
    'a|add'     => \( my $ADD    = undef ),
    'r|remove'  => \( my $REMOVE = undef ),
    'd|delete'  => \( my $DELETE = undef ),
);

unless ( defined $ADD or defined $REMOVE or defined $DELETE )
{
    pod2usage( -exitval => 1, -output => \*STDERR );
}

if ($ADD) {
   ...
}

if ($REMOVE) {
   ...
}

if ($DELETE) {
   ...
}
[download]

And you run the script with -

# the short form
script.pl -a -r -d
script.pl -a -d
...

# the long form
script.pl --add --remove --delete
script.pl --delete
...
[download]

Update: Here's a different approach - provide a list of commands to monitor on the commandline, separated by commas.

#!/usr/local/bin/perl -w
use Getopt::Long;
use Data::Dumper;
use strict;

# Parse command line arguments and assign corresponding variables
GetOptions
(
    'm|monitor=s' => \( my $MON = undef ),
);

unless ( defined $MON )
{
    print <<EOF
Usage: $0 <-m|--monitor [ADD][,REMOVE][,DELETE]>
Example:
  $0 -m ADD,DELETE
  $0 -m remove,delete
Note:
  No space between comma and commands
EOF
;
    exit(1);
}

my @options = map { uc } (split /,/, $MON);

# verify input commands
my @allowed_commands = qw/ ADD REMOVE DELETE /;
foreach my $opt (@options) {
    die "Unknown command $opt"
        if ! scalar grep $opt eq $_, @allowed_commands;
}

my $monitor_commands = join '|', @options;

# DEMO - give it a little test in action monitor...
# @actions simulates a sequence of commands
my @actions = qw/ ADD ADD REMOVE ADD DELETE FREE /;

foreach my $action (@actions) {
    if ($action =~ /($monitor_commands)/) {
        print "Command [$action] is monitored\n";
    } else {
        print "Command [$action] is NOT monitored\n";
    }
}
[download]

And the output for script.pl -m add,delete is -

Command [ADD] is monitored
Command [ADD] is monitored
Command [REMOVE] is NOT monitored
Command [ADD] is monitored
Command [DELETE] is monitored
Command [FREE] is NOT monitored
[download]

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