@stack = ( ['cfif', 'bool eq "true"'],
           ['cfelse']
         );

##</code><code>##

sub start {
 return if $stack[-1][0] eq 'cfif' and $tag ne 'cfelse' and not istrue($stack[-1][1]);
 return if $stack[-1][0] eq 'cfelse' and istrue($stack[-2][1]);
}
sub end {
 return if $stack[-1][0] eq 'cfif' and $tag ne 'cfif' and not istrue(...);
 return if $stack[-1][0] eq 'cfelse' and istrue(...);
}
sub text {
 something like what i used in start()
}

##</code><code>##

2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$