Simply because it gives you a free loop. I first saw this idiom about 10 years ago for decoding a cgi params into a hash. Something like

$p =~ s/(.*?)=(.*?)(&|$)/$v=$2;$v=~s/%(..)/chr(hex($1))/ge;$p{$1}=$v/g
+e
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a substitution within a substiution!

I also thought it might be quicker but after a quick benchmark it seems not.

I suppose I could have used

[/(\d+):(\d+)(?:,|$)(?{$hash{$1}=$2;push(@array, $1);1})/g];
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for a free loop without destroying the string but isn't much faster.