(Hint: do not use the /o option for regexes when doing this. You'll be unpleasantly surprised.)

./monitor.pl -add -remove
[download]

./monitor.pl -action ADD -action REMOVE
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

#!/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]

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

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

#!/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]

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]

You could use a hash of coderefs:

my %actions = (
    ADD = \&add,
    DELETE = \&delete,
    REMOVE = \&remove,
);

# ...

if($action =~ /$monitoredActions/) {
    $_ = uc($action);
    if(exists $actions{$_}) {
        $actions{$_}->();
    } else {
        # undefined action. die/warn whatever
    }
}

sub add { ... }
sub delete { ... }
sub remove { ... }
[download]

Update: had misunderstood question.

# Figure out what actions the user wants to monitor.
my $monitored_actions = join "|", @ARGV;

# Create a monitoring RE that can be used again and again
# without re-compiling it.
my $monitor = qr/$monitored_actions/;

{
    # Decide what action to take.
    my $action = ('ADD', 'REMOVE', 'DELETE', 'QUIT')[rand(3)];

    # And here we monitor it.
    $action =~ /$monitor/ && do { print $action, "\n"; };

    redo unless $action eq 'QUIT';
}
[download]

Then run it from the command line like this:

./mytest ADD REMOVE
[download]

TI(always)MTOWTDI

Here you go, sans if/else:

$actions = "ADD|JUNK|DEL";
sub{(${{
     ADD => sub{print "Adding\n"},
     DEL => sub{print "Deleting\n"},
     REM => sub{print "Removing\n"},}}{$_}
     or     sub{print "I'm Sorry Dave. I can't do <$_>.\n"}
)->() }->() for split/\|/,$actions;
[download]