package MyClass;
use strict;
use vars qw/$MUSTHAVEONLYONEINSTANCE/;
$MUSTHAVEONLYONEINSTANCE = 0;
...

sub new {
   return undef if $MUSTHAVEONLYONEINSTANCE;
   $MUSTHAVEONLYONEINSTANCE = 1;
   ...
}

sub DESTROY {
   $MUSTHAVEONLYONEINSTANCE = 0;
}