With the plugin installed, opening *.psgi files causes some special things to happen.
PSGI files are really just ordinary Perl files, so Padre does its normal Perl lexing/syntax highlighting magic on them, but the real fun starts with the Plack-specific features that appear in the per-file graphical plackup control panel that shows up.
The panel lets you run your web app in a Plack server at the click of a button, view server output, configure plackup options and launch a web browser on the appropriate port.
Patrick Donelan released 0.04 of Plack plugin for Padre, the perl IDE. Read more about the plugin announced earlier on his blog.