use Loop::Watch;

my $x = 2;
ensure { $x > 0 } watching($x), doing {
  # all your code centered around $x here
};

##</code><code>##

package Tie::Scalar::Watch;

use Carp;
use strict;

# rich man's Exporter ;)
# this just puts 'watch()' in the caller's namespace
sub import {
  my $pkg = caller;
  no strict 'refs';
  *{ $pkg . "::watch" } = \&watch;
}

sub TIESCALAR {
  my ($class, $val, $rule) = @_;
  my $self = bless [ undef, $rule ], $class;
  $self->STORE($val);
  return $self;
}

sub FETCH { $_[0][0] }

sub STORE {
  my ($self, $val) = @_;
  return $self->[0] = $val if $self->[1]->($val);
  croak "Value $val is out of bounds";
}

sub watch (&$) {
  tie $_[1], __PACKAGE__, $_[1], $_[0];
}

1;

##</code><code>##

use Tie::Scalar::Watch;  # imports 'watch'

my $x = 10;
watch { $_[0] > 5 } $x;

# $x is now transparently tied!
# and you thought tying was transparent already!

while (1) {
  print "$x\n";
  $x--;
}

##</code><code>##

sub STORE {
  my ($self, $val) = @_;
  $self->[0] = $val;
  $self->[1]->() ?
    return $val :
    croak "Value $val is out of bounds";
}