use warnings;
use strict;

sub work {
    my ($x, $y, $callback) = @_;

    if (ref $callback ne 'CODE'){
        die "callback param needs to be a code reference";
    }

    $callback->($x, $y);
}

# with a ref to an existing sub

sub this {
    my ($x, $y) = @_;
    print "$x $y\n";
}

work(3, 4, \&this);

# with an explicit code ref

my $cref = sub { print "$_\n" for @_; };
work(2, 2, $cref);

# or inline

work(2, 3, sub { print "$_[0], $_[1]\n"; })

##</code><code>##

my $regex = qr/$self->{regex}/;
while(<$input>){
    my $result = /$regex/;
        $self->{callback_ref}($result,$line,$output);
}