Everything POE does is legal Perl.

=> is one of Perl's comma operators. See the perlop man page (perldoc perlop) and search for the "Comma Operator" section. Or read about Perl's comma operators online.

This is a weird way of passing parameters.

As discussed in rcaputo's reply and the link(s) therefrom, it is another (and exactly equivalent) way of writing
    $kernel->post('ua', 'request', 'got_response', GET $url);
except, IMO, less messy (for some definition of 'mess') and distracting.

Another manifestation of this idiom is in the quasi-self-documentating
    return bless $objref => $class;
which one might find in an object constructor and which might be read as 'bless object reference into class'.

It had never occurred to me to use the automatic quoting of barewords before the fat comma for anything other than a hash key. Apparently it works in the even-numbered list positions as well, but I find this usage confusing.

Given this power of the fat comma, you could write

use strict;
my @days = (Mon=>Tue=>Wed=>Thu=>Fri=>);
[download]

The fat comma is recommended for use in natural pairs, mostly the key-value pairs of a hash, and the option to omit the quotes around the hash keys is widely thought to improve the presentation.

Update: I stand corrected by the response below. Given that the post() parameters are not pairs, the following equivalent code would be standard formatting:

$kernel->post(
    ua           => 'request',
    got_response => GET $url,
);
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$kernel->post(
    'ua',
    'request',
    'got_response',
    GET $url,
);
[download]

Except that the post() parameters aren't pairs, and grouping them that way misleads the reader. Typography is significant to the human audience.

--
Rocco Caputo - http://poe.perl.org/ - irc://irc.perl.org/poe