Here is the code with the module:
You can use different names of operation and subs here.

package Register;
use strict;

sub new {
    my $class = shift;
    my $self = {_register => 0 };
    bless $self, $class;
}

sub set {
    my $self  = shift;
    $self->{_register} = shift;
}
sub add {
    my $self = shift;
    $self->{_register} += shift;
}
sub multiply {
    my $self = shift;
    $self->{_register} *= shift;
}
    
sub value {
    my $self = shift;
    return  $self->{_register};
}

package main;

my $r = new Register;
my $ops = { set => 'set',
         add => 'add',
         mul => 'multiply',
         };

while(<DATA>){
    s/^\s+//;
    s/\s+$//;
    
    my ($operation,$param) = split;
    my $sub = $ops->{$operation};
    $r->$sub($param);
    print $r->value,"\n";
}

__DATA__
    set 5
    add 2
    mul 2
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