The posted code actually has a sort of cascade effect; it starts at the highest-numbered (last) value, subtracts that from the second-last, then moves down one, subtracting that from the one below, etc: 
pdl> p $x = floor random(5) * 10
[9 4 3 2 8]
pdl> p Dumper($xperl = $x->unpdl)
$VAR1 = ['9','4','3','2','8'];
pdl> $xperl->[ $_ -1 ] -= $xperl->[ $_ ] for reverse 1 .. $#$xperl;
pdl> p Dumper($xperl)
$VAR1 = [14,-5,9,-6,8];

[download]
The way to achieve that would be to do the below, but multiplying $to_sub by sequence($x->dim(0)) before subtracting. On the assumption this was not intended and instead the idea was to rotate the vector one over, then subtract that from the original: 
pdl> p $x = pdl '[9 4 3 2 8]'
[9 4 3 2 8]
pdl> $to_sub = $x->rotate(-1)->sever # MUST sever, or next mutates $x
pdl> $to_sub(-1) .= 0
pdl> p $to_sub
[4 3 2 8 0]
pdl> p $x - $to_sub
[5 1 1 -6 8]

[download]

In reply to Re: PDL: efficient self-referencing math? by etj
in thread PDL: efficient self-referencing math? by BrowserUk

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