It works by specifying perl snippets (most useful is a single "s///" operator) that modify $_ as resources:
URxvt.selection-autotransform.0: transform URxvt.selection-autotransform.1: transform ...
For example, the following will transform selections of the form "filename:number", often seen in compiler messages, into "vi +$filename $word":
URxvt.selection-autotransform.0: s/^([^:[:space:]]+):(\\d+):?$/vi +$2 \\Q$1\\E\\x0d/
And this example matches the same,but replaces it with vi-commands you can paste directly into your (vi :) editor:
URxvt.selection-autotransform.0: s/^([^:[:space:]]+(\\d+):?$/:e \\Q$1\\E\\x0d:$2\\x0d/
Of course, this can be modified to suit your needs and your editor :)
To expand the example above to typical perl error messages ("XXX at FILENAME line YYY."), you need a slightly more elaborate solution:
URxvt.selection.pattern-0: ( at .*? line \\d+[,.]) URxvt.selection-autotransform.0: s/^ at (.*?) line (\\d+)[,.]$/:e \\Q$1\E\\x0d:$2\\x0d/
The first line tells the selection code to treat the unchanging part of every error message as a selection pattern, and the second line transforms the message into vi commands to load the file.