package Foo::Bar;

use Regexp::Assemble;

our $VERSION = '0.01';
my $regex;

sub new {
    my $class = shift;
    my $self = bless({@_}, $class);

    return $self;
}

sub function {
    my $self;

...

    $regex ||= Regexp::Assemble->new->add( 'some-stuff' )->re;
    $var =~ $regex;
}