A simple approach here is to do:

our $i //= 28;

That is still declare it as a package global variable to work around strict but only assign 28 if not already defined.

A simple approach here is to do:

our $i //= 28;

That is still declare it as a package global variable to work around strict but only assign 28 if not already defined.

Alternatively, with zsh's zmv:

autoload -Uz
n=28; zmv -n '(*_)<->(.png)' '$1${(l[3][0])$((n++))}'

(like for most implementations of rename, -n is for dry-run).