> I'm not sure how I could make it clearer in the documentation that all parameters to ->content_as_png are optional, most likely I'll add a reference to screenshot.pl in the next release. Other suggestions are welcome!

what about adding

All parameters are optional. 
 
COORDINATES default to whole inner (?) window.
(incomplete COORDINATES result in a warning/error)
 
TAB defaults to current TAB.
[download]

I'm not sure about the "inner" or "outer", I suppose you can't capture the outer frame-borders...

UPDATED about incomplete coordinates.

UPDATE: Or just change the first line to

Without parameters returns the given tab or the current page rendered as PNG image."

UPDATE:

wouldn't the head definition better be

$mech->content_as_png [TAB] [, COORDINATES]

instead of

$mech->content_as_png [TAB, COORDINATES]

implying that one can define just the TAB independently from COORDINATES?

I'm not sure about this [optional arg] notation, is there a formal definition for POD?

In perl context it pretty much looks like arrayrefs which confuses me sometimes.

UPDATE: I'd rather prefer parens around args, which is a matter of taste

$mech->content_as_png ( [TAB] [, COORDINATES] )

... but you sometimes inconsistently used parens for signatures.

like in:

$mech->synchronize( $event, $callback )