Looks like it is trying to parse key value pairs that are delimited by ampersands ... maybe GET parameters? [^&]+ matches anything that is not an ampersand and the parens around it capture the match into $1. \& matches a mandatory ampersand. Here is some test code to see what is matched and what is not: 
use strict;

for (qw(foo foo& &foo foo&bar)) {
    /([^&]+)\&/;
    print "$_ - $1\n";
}

[download]
Seems to me that the variable $key in your code should be renamed $value, because that is what it is really trying to capture. Also, depending upon what the goal really is for this code, i might suggest using a CPAN module such as URI to pull out the key/value pairs: 
use strict;
use URI;

my $uri = URI->new();
$uri->query('THING=super%20hero&KEY=foobar');

my %hash = $uri->query_form();

print $hash{KEY}, "\n";

[download]
Notice that KEY is last, and it's value is not succeeded by an ampersand.

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

In reply to (jeffa) Re: A regex I cant get my head around by jeffa
in thread A regex I cant get my head around by Anonymous Monk

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