This is dangerous: the nature of my $var = ... if ... is predictable but very noninuitive (it acts something like a state variable). It has long been regarded as a bug that perl core would dearly love to fix, but cannot due to back-compat.

I recommend always either splitting out the declaration from the assignment:

my $f;
$f = $h->{b}{c}{d} if exists($h->{b}{c}) and exists($h->{b}{c}{d});
[download]

.. or using state explicitly when that's what you actually intend:

use feature 'state';  # 'use 5.10' or later also gives this automatica
+lly
state $f = $h->{b}{c}{d} if exists($h->{b}{c}) and exists($h->{b}{c}{d
+});
[download]